lo.
BW6CENCES LIBRARY
Agric.- Genetics
v,
xos^e V
Contents
Studies on chromosomes
I. The behavior of the idiochromosomes in
Hemiptera.
II. The paired Mi crochromo somes , idiochromo-
somes and heterotropic, chromosomes in
Hemiptera.
III. The sexual differences of the Chromosome-
groups in Hempitera, with some considera-
tions of the determination and inheritance
of sex.
IV. The "accessory" chromosome in Syromastes
and Pyrrochoris with a comparative review
of the types of sexual differences of the
chromosome groups.
V. The chromosomes of Metapodius, a contribu-
tion to the hypothesis of the genetic con-
tinuity of chromosomes.
VI. A new type of chromosome combination in
inoteipha.se.
VII. A review of the chromosomes of Nezara;
with some more general considerations.
VIII. Observations on the maturation- phenome-
na in certain Hempitera and other fonns
with considerations on synapsis and re-
duction.
967569
E
With the compliments of
EDM, B, WILSON,
COLUMBIA UNIVERSITY, NEW YORK,
STUDIES ON CHROMOSOMES
THE BEHAVIOR OF THE IDIOCHROMOSOMES
IN HEMIPTERA
By
EDMUND B. WILSON
RETURN TO
DIVISION OF GENETICS
HILGARD HALL , .
REPRINTED FROM
THE JOURNAL OF EXPERIMENTAL ZOOLOGY
Volume II
No. 3
BALTIMORE, MD., U. S. A.
August, 1905
STUDIES ON CHROMOSOMES.
I. THE BEHAVIOR OF THE IDIOCHROMOSO.MES IN
HEMIPTERA. 1
EDMUND B. WILSON.
WITH 7 FIGURES.
In studying the spermatocyte-divisions in Lygaeus turcicus and
Coenus delius, and afterward in several other genera of Hemip-
tera, my attention was directed to the fact that the number of
chromosomes appeared to vary, polar views of the equatorial
plate showing sometimes seven chromosomes," sometimes eight
(cf. Figs. ib y it, 2a, 2i, 3). For this reason no clue to the identification of the idiochro-
mosomes is given by their position. Two of the chromosomes are,
however, distinctly smaller than the others; and the relations in
the spermatogonia leave little doubt that it is these two that corre-
spond to the idiochromosomes. They may lie side by side (Fig.
4*3) or more or less widely separated (4^). All these chromosomes
are, in side view, seen to have a symmetrical dumb-bell shape, and
all are equally halved in the first division. None of the prepara-
tions show the stage immediately following; but there can be no
doubt that a conjugation of the two small chromosomes takes
place at this time, since the second division (of which I have a large
number) invariably shows in polar view but 7 chromosomes, which
have now assumed the usual arrangement, with one in the center
of a ring formed by the 6 others (Fig. 4). In
stage d (Fig. 6) when all the chromo-
somes are massed together, the large idiochromosome still unmis-
takably appears as a very distinct deeply staining rod, sometimes
nearly straight, more usually curved, and frequently horse-shoe
shaped, at one side of the chromosome-mass (6#). Neither
plasmosome nor small idiochromosome can now be made out.
In Coenus at this period (Fig. 50) both idiochromosomes (or the
single chromosome-nucleolus) still appear as compact, deeply
staining spheroidal bodies, the larger one typically having a small
plasmosome attached to it at one side.
The early history of the large idiochromosome proves most
clearly that the chromosome-nucleolus into which it afterward
condenses is a modified chromosome (as Montgomery first showed
in Euschistus variolarius) and one that forms a connecting link
between the ordinary chromosomes and the more usual forms of
"chromatin-nucleoli" in Hemiptera, described by Montgomery,
or the accessory chromosome of Orthoptera; for in none of these
latter is the condensation to a compact body so long delayed.
Paulmier ('99) showed, however, that in Anasa the compact chro-
mosome-nucleolus of the synaptic period afterward elongates
considerably and appears as a rather short longitudinally split
rod, similar to that of Lygaeus at stage d (Paulmier, Fig. 22)
afterward again condensing into a compact tetrad. In Lygaeus
there can be little doubt that the central cavity of the spheroidal .
chromosome-nucleolus, often visible in stages e-g, represents the
original longitudinal split. It is therefore hardly open to doubt
that the division of the large idiochromosome in the first mitosis is
an equation-division, and the same is probably true of the small
'This has been somewhat exaggerated in the engraving.
392 Edmund B. Wilson.
idiochromosome. It is, on the other hand, quite certain that the
division of the idiochromosome-dyad in the second mitosis is a
reduction-division. The order of the divisions in case of the
idiochromosomes is thus the reverse of that which occurs in the
other chromosomes, according to Paulmier's and Montgomery's
accounts; and as pointed out beyond, it is also the reverse of that
which takes place in the division of the small central chromosome
in Anasa and Alydus.
As already stated, I did not in Lygaeus and Coenus, succeed in
rinding any certain explanation of the fact that the nuclei of the
growth-period may show either one or two chromosome-nucleoli.
In Brochymena, however, there is very clear evidence on this
point. Here, too, the nuclei of the middle growth period show
either one or two spheroidal chromosome-nucleoli, the former con-
dition being much the more frequent. When both are present
they may be widely separated or close together, and both very
clearly show a central cavity, which is rendered very conspicuous
by the fact that the chromatin is frequently concentrated in a
dark zone immediately around it (Fig. *]c-g). When but one
is present it is, as a rule, perfectly spherical, hollow, and shows no
evidence of a double nature (Fig. 7/, g]. In the early growth-
period, however, the single chromosome-nucleolus almost always
appears bipartite, being composed of two unequal halves, forming
an asymmetrical dyad (Fig. Ja, &) very similar to that seen in the
second maturation-division (Fig. yra). At a later period both
of the constituents become hollow (and hence appear somewhat
larger) and stain less deeply; and all gradations may be observed
in the fusion of the two bodies to form a single hollow' body (Fig.
Jc, d, /) which is plainly as large as the two separate chromosome-
nucleoli (such as may be seen in cells of the same cyst) taken
together. In Brochymena, therefore, there can be no doubt that
when only one chromosome-nucleolus is present it is to be considered
as a bivalent body arising by the fusion or synapsis of the two
idiochromosomes.
Thus far the facts confirm the interpretation given by Mont-
gomery ('01, i) who observed in Coenus, Euschistus tristigmus
and some other forms that the cells of the growth-period may show
either a single "chromatin-nucleolus" or (in Euschistus "appar-
ently more frequently") two such bodies that are unequal in size;
and this fact he interpreted to mean that the two corresponding
Studies on Chromosomes.
393
spermatogonial "chromatin-nucleoli" may either conjugate at the
period of general synapsis to form a bivalent body or may remain
separate as univalent bodies. As already pointed out, it was
probably this interpretation that led him to conclude that the
first division in these forms might show either seven or eight
chromosomes. But the later stages observed in Brochymena
give conclusive evidence that even though such a primary synapsis
* :\ i i
09 m
o
FIG. 7.
Brochymena, Trichopepla. a-n, Brochymena, o-r, Trichopepla; a, b, idiochromosomes and plas-
mosomes, from early growth-period; c-g, condition of the idiochromosomes in middle and late growth-
periods; h, prophase of first division, showing idiochromosome-tetradj i, j, two stages, one following
and one preceding the last, in the division of the bivalent chromosome-nucleolus; k, metaphase-group,
first division; /, metaphase-group, second division; tn, side view, second division, showing idiochromo-
some-dyad and two other chromosomes; n, spermatogonial metaphase-group; o, metaphase-group, first
division, Trichopepla; p, spindle of second division in lateral view; q, r, sister-groups, late anaphase of
second division.
394 Edmund B. Wilson.
of the idiochromosomes takes place the bivalent chromosome-
nucleolus again separates into its univalent constituents in the early
prophases of the first maturation-division. This process, which
at first greatly puzzled me, occurs at the time just preceding the
concentration of the larger chromosomes into their final condensed
form (corresponding to stage h in Lygaeus). In cysts of this period
every stage may be seen in the transformation of the single chro-
mosome-nucleolus into an asymmetrical tetrad, consisting of two
symmetrical dumb-bell shaped bodies of unequal size (Fig. 77), and
the separation of these unequal dyads to form the idiochromo-
somes of the first division (cf. Figs, jh, /, y). It is evident that
in these cases the final reunion or conjugation at the end of the
first division is not a primary synapsis, but a secondary process. 1
The facts observed in Brochymena make it very probable that
when only a single chromosome-nucleolus is present in Lygaeus,
Coenus and the other forms, it is there also a bivalent body as
Montgomery assumed; but the uniform separation of the idio-
chromosomes in the first division of all the eight species I have
examined is almost a demonstration that in all the forms a division
of the bivalent body must occur. On the other hand it seems
equally certain that in many of these forms the idiochromosomes
may fail to unite at the period of general synapsis, and may remain
separate through the whole growth-period; and in Brochymena
the same cysts that show the division of the single idiochromo-
some-tetrad may also contain nuclei in which the dumb-bell
shaped idiochromosomes are widely separated. In such cases it
seems probable that the conjugation of the idiochromosomes at
the close of the first spermatocyte-division must be regarded as a
true or primary synapsis that has been deferred to this late period.
DISCUSSION OF RESULTS.
The most essential fact brought out by a study of the idiochro-
mosomes is that in Lygaeus, Ccenus, Podisus, Euschistus, Brochy-
mena and Trichopepla a dimorphism of the spermatozoa exists,
there being two groups equal in number, both of which contain
'The division of the bivalent chromosome-nucleolus is similar to the process described by Gross
('04) in Syromastes; though it occurs at a much later period. For reasons that will appear in a
subsequent paper, I am, however, skeptical in regard to Gross's conclusion which is based on a study
not of the idiochromosomes but of paired microchromosomes similar to those of Anasa or Alydus.
. Studies on Chromosomes. 395
the same number of chromosomes, but differ in respect to one of
them. In this respect these genera differ from all those that
possess an accessory chromosome (Pyrrochoris, Anasa, Alydus,
etc.), since in the latter case one-half of the spermatozoa receive
one chromosome fewer than the other half. It is remarkable
that two types of dimorphism apparently so different should
coexist within the limits of a single order of insects. We are thus
led to inquire into the relation between the idiochromosomes and
the accessory; and this inquiry must also include the "small
chromosomes" of Paulmier and the "chromatin-nucleoli" of
Montgomery. It will conduce to clearness if the second part of
this question be considered first.
It is evident from the figures and descriptions of Montgomery
('98, '01, i, '01, 2, '04) that the bodies I have called idiochromo-
somes are identical with some of those that this author has de-
scribed under the name of "chromatin-nucleoli" (which are
usually also small chromosomes or microchromosomes); but it is
now manifest that the bodies described under the latter name are
not all of the same nature. It is evident that two types of these
bodies may be distinguished that differ markedly in their behavior
during the maturation-mitosis. One of these, typically repre-
sented in Anasa, Alydus and Protenor appear in the spermat-
ogonia in the form of two equal microchromosomes ("chromatin-
nucleoli") which like the other chromosomes sooner or later unite
in synapsis to form a bivalent body that lies at the center of the
equatorial plate of the first mitosis. Both divisions accordingly
show exactly one half the spermatogonial number of chromosomes
but it is a very noteworthy fact that the final conjugation of the
two microchromosomes is long deferred, taking place in the propha-
ses of the first division (as was first observed by Montgomery in Pro-
tenor and some other forms, more recently by Gross in Syromastes
and by myself in Anasa and Alydus). In this case there seems to
be no doubt that the first division of the bivalent body thus formed
is a reducing division. It appears to be further characteristic of
this type at least in all the forms mentioned that a true acces-
sory chromosome is associated with the microchromosomes, and
that only one-half of the spermatozoa receive one-half the somatic
number of chromosomes, the other spermatozoa receiving one
less than this. The distinction between the accessory and the
microchromosomes or "chromatin-nucleoli" first demonstrated
396 Edmund B. Wilson.
by Montgomery ('01) in Protenor, has been more recently shown
by Gross ('04) to exist in Syromastes; and I have also been able to
demonstrate the same fact in Alydus and in Anasa (Paulmier
having been in error in his identification of the accessory with the
small chromosome in the last-named form).
The second type includes the idiochromosomes, which in the
forms I have studied differ from the Anasa type in four respects
(Nezara being an exception in regard to the first of these), namely,
(l) in their unequal size, with which is correlated the fact that only
a single microchromosome appears in the spermatogonia; (2) in
the fact that the final conjugation or synapsis of these bodies is
deferred until the prophases of the second division, a result of
which is that the first division shows one more than half the sper-
matogonial number of chromosomes while the second division
shows exactly half the spermatogonial number; (3) in the fact that
in case of the idiochromosomes it is manifestly the second division
that is the reducing one, while my observations on Lygaeus render
it practically certain that the first is an equation-division; (4) in
the fact that no accessory chromosome in the usual sense is present
and all the spermatozoa receive the same number of chromosomes.
In view of these differences it seems expedient for the present to
place these two types in different categories.
It is evident from Montgomery's figures and descriptions that
he observed many of the details of the phenomena described in the
present paper; but it is equally clear from the varying interpreta-
tions that he adopted that he failed to reach any consistent general
result regarding the behavior of the idiochromosomes, or to recog-
nize the dimorphism of the spermatozoa. For example, it is
evident, I think, from his descriptions of Euschistus variolarius,
E. tristigmus, Coenus delius, Oncopeltus fasciatus, and Lygus
pratensis, that the essential facts in these forms agree with those
I have described, the idiochromosomes being in the last named
two species of equal size, as in Nezara; but Montgomery offers
for each of these cases a different interpretation. In the first
named species the idiochromosomes are clearly figured in his first
paper ('98, Figs. 171, 188, 189, etc.), and in Fig. 214 they are
shown separating in quite typical fashion in the second division
(the smaller one designated as a "chromatin-nucleolus"); but it
is evident from the descriptions given in both this and the following
paper ('01, i) that he did not reach a correct interpretation of the
Studies on Chromosomes. 397
facts. 1 In Euschistus tristigmus and Coenus delius the first divi-
sion is stated to show either seven or eight chromosomes (the
spermatogonial number being 14), but quite different interpreta-
tions are given of this in the two species, the conditions in Euschis-
tus being assumed to be due to the frequent failure of the two
"chromatin nucleoli" to unite in synapsis, while in the case of
Coenus seven of the chromosomes (including the large "chromatin-
nucleolus") are assumed to be bivalent, while the eighth is
an additional small "chromatin-nucleolus" not distinguishable
O
in the spermatogonia ('01, I, p. 166). In Euschistus tristigmus
the "chromatin-nucleoli" are stated to be of unequal size and to
be separated from each other without divison in the second
mitosis. This is evidently the same phenomenon that I have
described; though Montgomery overlooked the conjugation of the
two unequal "chromatin-nucleoli" at the end of the first division,
and expressly states that they are not joined together in the second
division. In Oncopeltus, likewise, the first division shows one
more than half the spermatogonial number, 16 (i. e., nine instead
of eight, precisely as I have described in Podisus), and this is
stated to result from the persistence of the two "chromatin-nucle-
oli" throughout the whole growth period without union; but an
interpretation differing from both the foregoing is here sought in
the assumption that each of the two "chromatin-nucleoli" is
bivalent, even in the spermatogonia ('01, I, p. 186). In Lygus
pratensis, finally, the first division shows 18 chromosomes and the
second 17, the still different explanation being here offered that
the two "chromatin-nucleoli " pass undivided one to each pole of
the first spindle ('01, i). Of these various interpretations only
the one given in the case of Euschistus tristigmus, I believe, con-
'The first mitosis is here clearly shown to have eight chromosomes, grouped in the same way as in
my "Euschistus sp." and the anaphase daughter-plate of the second division is shown with seven (Fig.
220), precisely as in the two species I have studied. Montgomery gave the spermatogonial number,
correctly I believe, as 14. He nevertheless concluded that all of the eight chromosomes (seven chromo-
somes + i "chromatin nucleolus'') divide separately in both divisions, apparently overlooking the fact
that this would give the spermatozoa one chromosome too many (since he himself demonstrated that the
"chromatin-nucleolus"' is a modified chromosome). This account of the divisions is not modified in
the paper of 1901 except in the statement that "in the second maturation-division the chromatin-nucleo-
lus is not always divided" (p. 161), while the spermatogonial number is now given as 16. Since the
figures of the earlier paper show that the divisions in E. variolarius are evidently the same as in the
species I have examined, I think that on both these points the first account was probably more accurate
than the later one.
398 Edmund B. Wilson.
forms to the true one, and it is probable that all of these cases
will be found to agree in the essential phenomena with those I
have determined in Lygaeus, Coenus, Nezara and the other forms.
We may now inquire what is the relation of the idiochromo-
somes to the accessory chromosome. The observations suggest
so obvious an answer to this question that I wish to indicate not
only the evidence in its favor, but more especially the difficulties
it has to encounter. In forms possessing an accessory chromo-
some the spermatozoa fall into two equal groups that differ only
in respect to one chromosome. The same is true of Lygaeus and
other forms that lack the accessory but possess the idiochromo-
somes, with the difference that in the former case the distinctive
chromosome is present in but one-half the spermatozoa, while in
the latter case two such distinctive chromosomes are present, one
of which is present in one-half, the other in the other half, of the
spermatozoa. It is impossible to overlook the evident analogy
between the two cases; and the idiochromosomes may in one sense
be considered as two accessory chromosomes that are never allotted
to the same spermatozoon since each fails to divide in the second
mitosis (precisely as is the case with the single accessory in other
Hemiptera). The difference between Lygaeus and Coenus in the
size-ratio of the idiochromosomes obviously suggests the view
that the single accessory of other forms may have arisen by the
disappearance of one of the idiochromosomes; and in Lygaeus the
smaller one is already so minute as distinctly to suggest a vestigial
structure. We might accordingly assume that in a more primitive
type the two idiochromosomes were of equal size (as in Nezara),
undergoing synapsis and subsequent reduction in the same way
as the other chromosomes; that Coenus and Lygaeus represent
successive stages in the reduction of one of these chromosomes;
and that by the final disappearance of the smaller one in such
forms as Anasa or Pyrrochoris a single accessory chromosome
remains.
This hypothesis at first sight seems to give a clear and intelli-
gible view of the origin of the accessory chromosome, and to recon-
cile the remarkable mode of spermatogenesis occurring in the
insects with forms in which no accessory seems to appear. But
further reflection shows that it has to encounter a formidable if
not insuperable difficulty in the fact that in some of the forms
possessing an accessory chromosome the number of spermato-
Studies on Chromosomes. 399
gonial chromosomes is an even one (as in Anasa and Syromastes);
and there seems to be no escape from the conclusion that the acces-
sory is here a bivalent body arising by the synapsis of two equal
spermatogomal chromosomes. Even in cases showing an odd
number of spermatogonial chromosomes (as in many Orthoptera
and some Hemiptera for example Alydus or Protenor) it has
been assumed, and with good reason, that one of the chromosomes
(probably the accessory) is already bivalent, 1 and Montgomery
has shown ('01, i) that in Protenor the large accessory ("chromo-
some x ") is sometimes transversely constricted into two equal
halves in the spermatogonia. A similar fact was subsequently
shown in Harmostes ('01, 2) which also has normally an odd sper-
matogonial number. To this should be added the fact that these
forms possess the small bivalent central chromosome (which
arises by the synapsis of two equal microchromosomes) in addition
to the accessory. The difficulty pointed out above cannot be
escaped by supposing that the disappearance of one of the idio-
chromosomes has been effected by its gradual absorption by the
other; for this assumption, too, fails to explain the even number
of spermatogonial chromosomes. Apparently therefore the hypo-
thesis I have suggested must in the present state of our knowl-
edge be considered untenable. 2
It appears more probable that the idiochromosomes are com-
parable to the two equal microchromosomes or "chromatin-
} Cf. Montgomery, '04.
2 Since this paper was sent to press I have determined beyond the possibility of doubt, I think, that
the number of spermatogonial chromosomes in Anasa tristis is 21, not 22 as given by both Paul-
mier and Montgomery. This result is based on the study of a large number of preparations, and
careful camera drawings of more than twenty perfectly clear metaphase figures have been made. All
without exception show 21 chromosomes, and I have sought in vain for even a single cell that shows
22. (Paulmier's original slides were used.) If corroboratory evidence be needed it is given by the
fact that there are always three macrochromosomes, one of which is obviously without a mate of like
size, and is probably the accessory. I have, also, positively determined the spermatogonial number
to be 21 in a form included in Paulmier's material and labeled " Chariesterus antennator,' ' (since
this number disagrees with Montgomery's co'unt of the spermatocytes there may be an error of iden-
tification; but the form is certainly different from Anasa) and 15 in Archimerus calcarator (from my
own material, identified by Mr. Uhler), both members of the same family as Anasa. This wholly
unexpected result perhaps justifies a certain skepticism in my mind in regard to the accounts of other
observers, who give an even spermatogonial number for forms possessing an accessory chromosome;
and if this be well founded the objection urged above disappears. I shall return to this subject
hereafter. It is needless to say that had I been acquainted with these facts, the discussion that
follows would have been different.
400 Edmund B. Wilson.
nucleoli" which in such forms as Anasa or Alydus conjugate to
form the small central chromosome of the first mitosis. The dif-
ferences between the two forms have already been pointed out.
Their resemblances are, however, no less obvious, namely, their
usual central position in the equatorial plate, small size, occasional
persistence as chromosome-nucleoli in the growth-period, and
their late conjugation. This comparison finds very definite support
in the conditions I have described in Nezara, where the idio-
chromosomes are of equal size and appear as two equal micro-
chromosomes in the spermatogonia. From the analogy of other
forms it is very probable that the more primitive and typical form
of synapsis is that between chromosomes of like size. It is there-
fore probable that such a condition as that observed in Nezara
is a less modified one than that in which the idiochromosomes are
unequal; and that the latter condition has arisen through a second-
ary morphological differentiation of two chromosomes that were
originally of equal size, and perhaps are represented by the two
equal microchromosomes that appear in the spermatogonia of
such Hemiptera as Anasa, Alydus, Syromastes or Protenor.
This comparison involves two assumptions, namely, first that in
case of the idiochromosomes the final conjugation of the micro-
chromosomes has been postponed from the prophases of the first
division to those of the second; and secondly that a reversal in the
order of the reduction- and equation-divisions has taken place in
case of these particular chromosomes, the first division being in
case of the Anasa-type the reduction- and in case of the idiochro-
mosomes the equation-division. The difficulty apparently involved
by the second assumption is less serious than may appear. All
the facts at our command indicate that a reduction-division
is the necessary, or at least invariable, sequel to a foregoing
conjugation; and if, as in the case of the idiochromosomes, the
final conjugation is deferred to the second division, the reduction-
division must also be deferred. The univalent idiochromosomes
as is shown with certainty in case of the larger one in Lygaeus
undergo longitudinal division at the same stage of the growth-
period as their bivalent companions and are already double
at the time of the first mitosis. There is, therefore, no difficulty
in the way of assuming indeed, the facts seem to admit of no
other conclusion that this is the equation-division.
It must be recognized, however, that the foregoing comparison
Studies on Chromosomes. 401
wholly fails to explain the origin or meaning of the accessory
chromosome, nor does it account for the surprising fact (of which
the phenomena in Brochymena seem to leave no doubt) that two
chromosomes may unite in synapsis, subsequently part company
so as to divide as univalents in the first mitosis, but again con-
jugate to form a bivalent in the second mitosis. It seems likely
that further comparative study of this phenomenon may throw
important light on the general mechanism of karyokinesis and
reduction.
The history of the idiochromosomes possesses a more general
interest in the strong support that it lends to the general theory of
the individuality of chromosomes, to the specific conclusions of
Montgomery and Sutton in regard to synapsis, and especially to
the correlation of the phenomena of reduction with those of Men-
delian inheritance attempted by the last-named author ('02, '03).
It has been assumed by some authors, including some of those who
have accepted Montgomery's remarkable conclusion ('01, i, '04)
that corresponding paternal and maternal chromosomes unite in
synapsis, that in this process the individuality of the conjugating
chromosomes is completely lost "Sie vereinigen sich zu einem
einzigen Zygosom, aus dem erst wieder zwei neue Chromosomen
hervorgehen." 1 It is undoubtedly true that frequently all visible
traces of the duality of the bivalents that emerge from the synapsis
stage are for a time lost; and as Sutton suggested ('03, p. 243), such
cases as those of first crosses that breed true and I may add,
perhaps also those in which blended inheritance or weakening of
dominance occurs may be taken to indicate that a permanent
fusion, or intermixture of the chromosome-substances, may really
take place. But, on the other hand, the history of the idiochro-
mosomes in cases where they remain separate through the whole
growth-period leaves not the least doubt that as far as these
particular chromosomes are concerned the same two that unite in
synapsis persist as distinct individuals to be afterward separated
by the reducing division and assigned to different germ-cells.
This preliminary conjugation and subsequent separation ensures
that the germ-cells shall be "pure" in respect to these particular
chromosomes /. ^., that both shall not enter the same spermat-
ozoon and if this be true of one pair of the conjugating chromo-
'Strasburger, '04, p. 26; cf. also Bonnevie, 'o
402 Edmund B. Wilson.
somes we have good reason to conclude that it may be true of all,
as Montgomery has urged and as Sutton has so cogently argued,
from a study of the size-relations. It is a fair working hypothesis
that the idiochromosomes represent a pair of corresponding or
allelomorphic qualities, or group of qualities, that are respectively
maternal and paternal, as Sutton, building on the basis laid by
Montgomery and himself, has argued for the chromosome-pairs
in general. The argument of Montgomery and Sutton is based,
it is true, on the fact that chromosomes of different sizes in the
spermatocytes are represented by symmetrical chromosome-pairs
of corresponding sizes in the spermatogonia; and to this the idio-
chromosomes in most of the cases described form an exception in
being unequal. If this appears to be a difficulty it is removed by
the case of Nezara, where the idiochromosomes are of equal size.
Even in the more usual case, where they are unequal, symmetrical
synapsis takes place between all the other chromosome-pairs.
If the theory of the individuality of chromosomes be granted no
other conclusion seems possible, accordingly, than that the remain-
ing two, despite their size-difference, are respectively the paternal
and maternal elements of the remaining pair; and if Sutton's
general hypothesis be well founded, these elements may be
assumed to be physiological correlates or allelomorphs. Their
marked difference in size suggests a corresponding qualitative
differentiation, and this inevitably suggests a possible connection
between them and the sexual differentiation. The visible dimor-
phism of the spermatid-nuclei in such forms as Lygaeus, Ccenus
or Podisus shows too obvious a parallel to the sexual dimorphism
of the germ-cells, indicated by so much of the recent work on sex-
determination, to be ignored; while in Nezara, where no visible
dimorphism exists, the spermatozoa nevertheless fall into two
equal groups in respect to the previous behavior of one of the
chromosomes. But such a suggestion as to the possible signifi-
cance of the idiochromosomes immediately encounters the diffi-
culty that both idiochromosomes are present in the male cells
(spermatogonia, and spermatocytes), just as McClung's similar
hypothesis regarding the accessory chromosome is confronted with
the fact, determined by Montgomery and Gross, that in the Hem-
iptera both sexes show the same number of chromosomes.
Whether these difficulties can be met by assumptions of dominance
and the like remains to be seen; but the fact should be recognized
Studies on Chromosomes. 403
that as far as the Hemiptera are concerned neither the suggestion
I have made, nor the hypothesis of McClung has at present any
direct support in observed fact. 1
The practical interest of the idiochromosomes lies in the very
definite basis that they give for an examination of the question by
the study of fertilization, for their disparity in size gives us the
hope of determining their history by direct observation. There
is good reason to believe that such a study will yield interesting
results.
SUMMARY OF OBSERVATIONS.
1. In Lygaeus turcicus, Coenus delius, Euschistus fissilis,
Euschistus sp., Brochymena, Nezara, Trichopepla and Podisus
spinosus all of the spermatids receive the same number of chromo-
somes (half the spermatogonial number), and no accessory chro-
mosome is present; but the spermatozoa nevertheless consist of
two groups, equal in number, which differ in respect to one of the
chromosomes, which may conveniently be called the "idiochromo-
some."
2. In all of the forms named, excepting Nezara, half the
spermatozoa receive a larger, and half a smaller, idiochromosome.
In Nezara the idiochromosomes are of equal size, but agree in
behavior with the unequal forms.
3. In all of the forms the idiochromosomes remain separate
and univalent in the first maturation-division, while the other
chromosomes are bivalent; this division accordingly shows one
more than half the spermatogonial number of chromosomes.
They divide separately in the first mitosis, but at the close of this
division their products conjugate to form a dyad, which in all the
forms save Nezara is asymmetrical. The number of separate
'The discovery, referred to in a preceding foot-note, that the spermatogonial number in Anasa is
21 instead of 22, again goes far to set aside the difficulties here urged. Since this paper was sent to
press I have also learned that Dr. N. M. Stevens (by whose kind permission I am able to refer to her
results) has independently discovered in a beetle, Tenebrio, a pair of unequal chromosomes that are
somewhat similar to the idiochromosomes in Hemiptera and undergo a corresponding distribution to
the spermatozoa. She was able to determine, further, the significant fact that the small chromosome is
present in the somatic cells of the male only, while in those of the female it is represented by a
larger chromosome. These very interesting discoveries, now in course of publication, afford, I think,
a strong support to the suggestion made above; and when considered in connection with the com-
parison I have drawn between the idiochromosomes and the accessory show that McClung's hypo-
thesis may, in the end, prove to be well founded.
404 Edmund B. Wilson.
chromatin elements is thus reduced to one half the spermatogonial
number. In the second maturation-division the asymmetrical
dyad separates into its two unequal constituents, the larger one
passing to one pole and the smaller one to the other pole of the
spindle, while the other dyads divide equally.
4. In all the forms excepting Nezara the spermatogonia
possess but one microchromosome (the small idiochromosome),
while in Nezara two equal microchromosomes are present as in
forms like Anasa which possess an accessory chromosome.
5. In the primary synapsis the idiochromosomes may unite to
form a bivalent body or may remain separate. In the former case
the bivalent body condenses to form a single chromosome-nucleolus
that persists throughout the whole growth-period, but again
separates into its univalent constituents before the first mitosis
(directly proved in Brochymena, inferred in the other forms).
If the idiochromosomes fail to unite in the primary synapsis,
they remain separate through the growth-period in the form of
chromosome-nucleoli. In either case the idiochromosomes divide
separately in the first mitosis.
6. In Lygaeus the large idiochromosome has in the synaptic
and early post-synaptic periods the form of a long longitudinally
split thread which afterward condenses into a hollow spheroidal
chromosome-nucleolus.
Zoological Laboratory, Columbia University,
May 5th, 1905.
WORKS CITED.
BONNEVIE, K., '05. Das Verhalten des Chromatins in den Keimzellen ente-
roxenos ostergreni. Anat. anz., xxvi, 13, 14, 15.
GROSS, J., '04. Die Spermatogenese von Syromastes marginatus; Zool. Jahrb.,
Anat. u. Ontog., xx, 3.
MONTGOMERY, T. H., '98. The Spermatogenesis in Pentatoma, etc. Zool.
Jahrb., Anat. u. Ontog., xii.
'01, I. A Study of the Chromosomes of the Germ-cells of Metazoa.
Trans. Amer. Phil. Soc., xx.
'01, 2. Further Studies on the Chromosomes of the Hemiptera heterop-
tera. Proc. Acad. Nat. Sci., Phil., March, 1901.
'04. Some Observations and Considerations upon the Maturation
Phenomena of the Germ-cells. Biol. Bull., vi, 3, Feb.
Studies on Chromosomes. 45
PAULMIER, F. C., '99. The Spermatogenesis of Anasa tristis. Jour. Morph.,
xv, supplement.
STRASBURGER, E., '04. Ueber Reduktionsteilung. Sitzber. Kon. Preuss. Akad.
Wiss., xviii, 24 Marz, 1904.
SUTTON, W. S., '02. On the Morphology of the Chromosome Group in Brachy-
stola magna. Biol. Bull., iv, I.
'03. The Chromosomes in Heredity. Biol. Bull., iv, 5.
WILSON, E. B., '05. Observations on the Chromosomes in Hemiptera. Rept.
N. Y. Academy of Sciences, May 8th, 1905; Science, xxi, 548,
June 30.
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k EDM, B, WILSON,
COLUMBIA UNIVERSITY, NEW YO
STUDIES ON CHROMOSOMES
II. THE PAIRED MICROCHROMOSOMES, IDIOCHRO
MOSOMES AND HETEROTROPIC CHRO-
MOSOMES IN HEMIPTERA
By
\r> B. WILSON
RETURN TO
DIVISION OF GENETICS
HILGARD HALL
REPRINTED FROM
THE JOURNAL OF EXPERIMENTAL ZOOLOGY
Volume II
No. 4
BALTIMORE, MD., U. S. A.
November, 1905.
STUDIES ON CHROMOSOMES.
II. THE PAIRED MICROCHROMOSOMES, IDIOCHRO-
MOSOMES AND HETEROTROPIC CHRO-
MOSOMES IN HEMIPTERA. 1
BY
EDMUND B. WILSON.
WITH 4 FIGURES.
In investigating the physiological significance of the chro-
mosomes and their individual values in heredity, it is important
to determine as accurately as possible how far they are differen-
tiated in respect to individual behavior, and to ascertain by the
comparative study of different forms to what extent the chro-
mosomes can be grouped in well-defined classes. The work of
Henking, Paulmier, Montgomery, Gross and Stevens on the Hemip-
tera has shown that this group is peculiarly favorable for such a
study; and I believe from my own observation that no group of
animals has thus far been examined that offers greater advan-
tages in this direction. 2 But although the general results obtained
by the above-mentioned observers are of great value and interest
they nevertheless show many discordances of detail that stand in
the way of a consistent general interpretation of the phenomena,
while some of Gross's conclusions are a stumbling block in the
way of the whole theory of the individuality of the chromosomes.
For this reason I propose in this paper to record a series of obser-
vations that I hope may serve to clear away some of the con-
fusion that now exists in the accounts of the subject, and that
open the way, I believe, to a true interpretation of the "accessory
chromosome" and its relation to the determination of sex.
In a series of suggestive papers ('01, '04, '05) Montgomery
'Attention is called to the Appendix in which are briefly recorded facts, determined by later
observations, that exactly realize the theoretic expectation regarding the sexual differences of the
chromosome-groups, stated at p. 539. An abstract of these observations was published in the issue of
Science for Oct. 20, 1905.
2 J am much indebted to Mr. Uhler's kindness in identifying many of the species examined.
508 Edmund B. Wilson.
has endeavored to bring together under the name of "hetero-
chromosomes" two classes of chromosomes in these insects,
namely, the "unpaired heterochromosome" ("accessory chro-
mosome" of McClung) 1 and the "paired heterochromosomes"
(or "chromatin nucleoli"), which differ markedly in behavior
from the other chromosomes during the maturation process.
Montgomery gives as the most essential characteristic of these
chromosomes "their difference in behavior from the other chro-
mosomes in the growth period of the spermatocytes and ovocytes,
as sometimes during the rest period of the spermatogonia, a dif-
ference which appears usually to consist in the maintenance of
their compact structure and deep-staining intensity, so that while
the other chromosomes become long loops or even compose a
reticulum, these do not undergo any such changes or only to
slight extent" ('05, p. 191). "Thanks to this peculiarity they
can be followed with extreme certainty from generation to genera-
tion, even during rest stages; and so are splendid evidence for the
thesis of the individuality of the chromosomes" ('04, p. 146).
The study of these chromosomes has led Montgomery to some
very important conclusions regarding synapsis and reduction with
which, as far as their more general features are concerned, I am
glad to find my own results in substantial agreement. Considered
more in detail, however, there are many points regarding which
I think Montgomery's general treatment of the "heterochro-
mosomes" requires emendation.
In a preceding paper (Wilson, '05) the fact was indicated that two
types of "paired heterochromosomes" or "chromatin nucleoli"
occur in Hemiptera. The first, including what I have called the
'Since there is no reason for considering the "accessory chromosome" as in any sense accessory to
the others, it appears to me that McClung's term might well be abandoned in favor of a less com-
promising one. I suggest that until their physiological significance is positively determined chro-
mosomes of this type may provisionally be called hete r ot'op;c chromosomes (in allusion to the fact that
they pass to one pole only of the spindle in one of the maturation-divisions) in contradistinction to
amphitropic chromosomes, the products of which pass to both poles in both divisions. There are several
objections to this term, one of which is that the "accessory'' chromosome behaves as a heterotropic
body in only one of the divisions (and probably in one sex only). Another is the fact that the members
("chromatids") of every chromosome-pair are heterotropic in the reducing division, since this only
separates univalent chromosomes that were previously in synapsis; but if, as in these studies, the term
"chromosome'' be consistently applied to each coherent chromatin-element of the equatorial plate,
whatever be its valence or mode of origin, this objection is perhaps not serious enough to weigh against
the convenience of the term.
Studies on Chromosomes. 59
"idiochromosomes" (which occur in such forms as Lygseus,
Euschistus, Coenus, Brochymena, etc.) are typically unequal in
size, and differ from all other known forms of chromosomes in
the fact that their union in synapsis gives rise to an unequal or
asymmetrical bivalent. The spermatogonial groups correspond-
ingly show but one small chromosome, since the larger idio-
chromosome is not noticeably smaller than the ordinary chromo-
somes. The second type includes the equal paired "chromatin
nucleoli" of such forms as Anasa, Alydus, Syromastes or
Archimerus. Since the latter are almost always markedly
smaller than the others they may conveniently be called the
paired microchromosomes, or better, in order to avoid all
ambiguity, simply the m-chromosomes; and these are distin-
guishable in the spermatogonial groups as an equal pair of
especially small chromosomes. The most obvious difference of
behavior between these two types, so far as is now known, is that
the idiochromosomes divide as separate univalents in the first
maturation-mitosis, which accordingly always shows one more
than half the spermatogonial number of separate chromatin
elements, while the m-chromosomes, like the other chromosomes,
always unite to form a bivalent before the first mitosis which
therefore shows the same number as in the second division.
Other no less characteristic differences are described beyond.
These two forms are not yet known to coexist in the same species;
and, as a rule, forms that possess the idiochromosomes do not
have an "accessory" or heterotropic chromosome, while as far
as now known such a chromosome is always associated with the
m-chromosomes.
The confusion that has grown out of the failure to observe these
differences arose in the first instance from two conclusions both of
which I shall show to be untenable reached by Paulmier in his
valuable, and, as far as the general history of the maturation-
process is concerned, very accurate, study of the spermatogenesis
of Anasa tristis ('99), and was increased by the subsequent efforts
of Montgomery ('01, '04, '05) to reduce the behavior of the
"chromatin nucleoli" to a uniform scheme. Paulmier, who was
the first to reexamine the history of the "accessory" chromosome
since its discovery by Henking, was also the first to describe the
m-chromosomes (in Anasa) as two very small chromosomes of
equal size in the spermatogonial metaphase-groups. These two
510 Edmund B. Wilson.
small chromosomes, he believed, united in synapsis to form a
single condensed bivalent chromosome-nucleolus which persisted
throughout the growth-period of the spermatocytes and later gave
rise to the small central "tetrad" of the first maturation-mitosis.
He believed, further, that after an equal division of this small
"tetrad" in the first mitosis each of its products passed undivided
to one pole of the second spermatocyte-spindle. He therefore
compared the "small tetrad" (microchromosome-bivalent) of
Anasa to the body, first discovered by Henking in Pyrrochoris, and
afterward found in the Orthoptera and some other insects by
McClung and others, to which the last-named author gave the
name of "accessory chromosome." In identifying the chro-
mosome-nucleolus of the growth-period as the microchromosome-
bivalent Paulmier has been followed by Montgomery in all of
his papers and with some modifications by Gross ('04) in his recent
study of Syromastes. Paulmier's conclusion on this point cannot,
however, be sustained, as I shall try to show; and the same is true
of his identification of the microchromosome-bivalent as the
"accessory" or heterotropic chromosome.
I. GENERAL HISTORY OF THE M-CHROMOSOMES AND THE HET-
EROTROPIC CHROMOSOME DURING THE GROWTH-PERIOD AND
IN THE MATURATION-DIVISIONS.
The behavior of the m-chromosomes in the maturation-divisions
may conveniently be considered first.
Paulmier's original preparations, 1 as well as my own more recent
ones, give demonstrative evidence of the equal division of the
small central chromosome in both maturation-mitoses, and the
same appears no less clearly in Alydus and in Archimerus, pre-
cisely as has been shown by Montgomery ('01) in Protenor and
by Gross ('04) in Syromastes. I was long since led to suspect an
error in Paulmier's conclusion in regard to this point from the
fact, which clearly appears in his own figures, that the "accessory"
is nearly or quite as large as the other chromosomes, and much
larger than the products of the first division of the small bivalent.
J I have in the previous paper acknowledged my indebtedness to Dr. Paulmier's generosity in placing
at my disposal his entire series of preparations of Anasa and other insects. He has since added to this
indebtedness by sending me from time to time a large amount of valuable living material.
Studies on Chromosomes. 511
(Cf. Paulmier's Figs. 28, 34-36, and my Fig. 2, k-n.) Both in
Anasa and in Alydus careful search among longitudinal sections
of the second division shows in fact in the clearest manner, that
the "small dyad" divides into equal halves, so that each of the
spermatids received one of its products (Figs. I, i-m\ 2, m, n).
The heterotropic chromosome is a much larger body, as shown
by the figures, in Anasa fully equal in size to some of the larger
single chromosomes of the anaphases of the second division.
Paulmier's failure to observe the second division of the small
bivalent is easily explained by the difficulty of observing this body
owing to its usually central or subcentral position, and the mistake
was a very natural one at the time his paper was written. Had he
examined Alydus where there are but seven chromosomes, which
show marked and constant size-differences, he could not have
failed to observe this division.
We have now to examine a second and more difficult point,
namely, the nature of the condensed nucleolus-like body
(chromosome-nucleolus) of the growth-period, which so closely
simulates the heterotropic chromosome of the Orthoptera at the
corresponding period. I have always doubted Paulmier's and
Montgomery's conclusion that this body is the microchromosome-
bivalent, from the fact, clearly shown in the figures of both these
authors, that the chromosome-nucleolus of the synaptic and
growth-periods is always larger, and in some species very much
larger (e. g., in Alydus) 1 than the two spermatogonial micro-
chromosomes taken together, or than the small central bivalent
to which it was assumed to give rise. (Cf. Paulmier's Figs. 16-21,
with 26, 28.) This fact did not escape Montgomery's attention,
but he explained it as due to an increase of volume on the part
of the chromatin-nucleolus in the early growth-period and a
corresponding decrease in the late growth-period or in the pro-
phases of the first division ('01, p. 203). This explanation was,
however, not supported by any sufficient evidence; 2 and the only
detailed evidence on this point has been brought forward by
Gross ('04) in the case of Syromastes. This observer, however,
while apparently confirming Paulmier and Montgomery as to
l Cf. Montgomery, '01, Figs. 96-98.
^Montgomery's study of the facts in Euschistus ('98) is not in point, since he was here undoubtedly
dealing with the idiochromosomes and not with the w-chromosomes.
512 ' Edmund B. Wilson.
the fate of the chromosome-nucleolus, differs entirely from them
in regard to its origin, concluding that it is derived from two of
the larger spermatogonial chromosomes. In the attempt to
reconcile these contradictory results (with both of which my own
are in disagreement) he is led to some speculative conclusions that
I think must be regarded as highly improbable. 1
A careful study of all the intermediate stages, not only in Anasa,
but also in Alydus, Archimerus, and Chariesterus gives in point
of fact, evidence that I believe is quite decisive, that the small
central bivalent is not derived from the large chromosome-nucleolus
of the growth-period, and that the latter is nothing other than the
accessory or heterotropic chromosome, precisely as in the Orthoptera.
To the differences between the idiochromosomes and the m-chro-
mosomes already stated may therefore be added the fact that the
former, like the heterotropic chromosome, may form a single
chromosome-nucleolus during the growth-period, while this is
not the case, in the forms I have studied, with the m-chromosomes.
It may seem strange that Montgomery, after accurately tracing
the history of the heterotropic chromosome ("chromosome *")
in Protenor and showing its complete independence of the "chro-
matin-nucleoli" (m-chromosomes) was not led to suspect a
similar relation in the other forms. That he apparently did not
do so was doubtless due to his having failed to distinguish between
the m-chromosomes and the idiochromosomes, which latter bodies
he correctly identified (in'Euschistus, etc.) as the bivalent chromo-
some-nucleolus (or two separate univalents) of the growth-period.
The entire independence of the large chromosome-nucleolus
and the m-chromosomes is most obvious in Alydus and Archi-
merus, partly because in both these forms the heterotropic
chromosome is at every period recognizable by its characteristic
size, partly because in Alydus certainly, and I believe also in
Archimerus the m-chromosomes frequently assume a compact
condensed form at a much earlier period than in Anasa; they can
therefore be recognized in addition to the heterotropic chromosome,
throughout the latter part of the growth-period, at a time when
the larger chromosomes are still in the pale, vague condition
characteristic of so many of the Hemiptera at this period.
In Alydus pilosulus the first mitosis invariably shows seven
'Gross ('04) pp. 481, 482.
Studies on Chromosomes . 5*3
bivalent chromosomes, which show very marked and characteristic
size-differences (Fig. 'i, c-g, ). There are always (i) a largest
chromosome or macrochromosome, which is frequently quad-
ripartite; (2) a second largest; (3) three slightly smaller ones of
nearly equal size; (4) a fourth, considerably smaller than the last;
and finally (5) the smallest or michrochromosome-bivalent.
These show a characteristic grouping, the five larger ones forming
an irregular ring with the small bivalent ("chromatin nucleolus")
at its center, while the next smallest lies more or less at one side
of the ring (Fig. i, g}. In the first division all these chromosomes
are equally halved (Fig. i, /). In the second all are again halved
with the exception of the second smallest which passes undivided
to one pole of the spindle (Fig. i, /-o). The size-relations leave
not the least doubt that this chromosome is derived from the one
of corresponding size in the first division *. e., the odd or eccen-
tric one and the latter accordingly is to be identified as the
"accessory" or heterotropic chromosome. In the first division
this chromosome sometimes shows a quadripartite form (as was
described by Paulmier in Anasa) sometimes a dumbbell-shaped
or dyad-like form. In the second it is usually unconstricted and
often curved (Fig. I, z, ra, w), sometimes into a U-shape so as
almost to appear double (Fig. I, o).
A study of the growth-period shows that the heterotropic
chromosome may be traced uninterruptedly backward from the
metaphase of the first division to the contraction-phase of the
synaptic period, being always in the form of a condensed chro-
mosome-nucleolus, which in the early growth-period is attached
to a large, pale plasmosome, from which it afterwards separates.
It is impossible to mistake this chromosome, owing to the fact
that its characteristic size does not noticeably change except that
it becomes slightly larger as the growth-period advances (probably
owing to the presence of a central cavity), again becoming slightly
smaller as the general condensation takes place. (Cf. Fig. i,
a-c.~) In the contraction-phase (Fig. I, #) and in the early post-
synaptic spireme the m-chromosomes are not visible, but as the
larger chromosomes assume the peculiar pale, ragged, clumped
condition, characteristic of the middle and late growth-periods,
the m-chromosomes frequently come into view, in the form of
two compact, intensely-staining bodies, that may occupy any
relative position (Fig. I, &). The period at which these bodies
5H Edmund B. Wilson.
FIGURE I.
Alydus pilosulus. a, Contraction-phase of synaptic period, "accessory" (h) in the form of a con-
densed chromosome-nucleolus attached to a large plasmosome (/>); b, spermatocyte-nucleus, middle
growth-period, showing large diffused chromosomes "accessory" still attached to the plasmosome
and the two condensed m-chromosomes on opposite sides of the nucleus; c, early proph?se of first
division, showing all of the chromosomes, the larger ones condensing; d, late prophase, showing
"accessory'' (h) and the two w-chromosomes still separate; e, slightly later prophase, showing all
of the chromosomes; /, initial anaphase, first division, the w-chromosomes separating; g, polar view of
metaphase-group, first division; h, polar view of metaphase-figure, second division; /', j, initial
anaphases, second division; k, spermatogonial metaphase-group; /, m, n, o, anaphases of second
division.
Studies on Chromosomes.
5'5
FIGURE i. 1
] Thc figures are all drawn to the same scale as those of the preceding study.
516 Edmund B. Wilson.
condense into the compact form appears to vary considerably,
for they cannot always be distinguished until the later growth-
period, and it should be noted that during the pale period the
nuclei often show a variable number of smaller deeply-staining
granules. I believe, however, that there can be no doubt as to
the nature of the two larger bodies on account of their great con-
stancy, their size, and the completeness of the series that connects
the earlier with the later conditions (such as is shown in Fig. I, <:),
where no doubt of their nature can exist. The persistence of the
larger chromosome-nucleolus ("accessory") throughout all these
stages without any considerable change renders it manifestly
impossible that it should give rise to the ra-chromosome bivalent,
either directly as assumed by Paulmier and Montgomery, or by
division into two univalents that subsequently conjugate, as
described by Gross in Syromastes.
In the early prophases the larger chromosomes resume their
staining capacity and condense into characteristic cross-forms
(Fig. i, <:), and finally into compact quadripartite tetrads or bipar-
tite bodies. At this time the heterotropic chromosome assumes
a dumbbell or quadripartite shape, and the w-chromosomes,
which are still quite separate and may even lie on opposite sides
of the nucleus, also frequently become bipartite. The nucleus now
contains, accordingly, eight separate chromatin-elements, one more
than the number of bivalents in the first mitosis, as is also the case
in Archimerus and Anasa, as described beyond. As the spindle
forms the two microchromosomes lose their bipartite shape,
approach each other, and in the stage just preceding the metaphase
finally conjugate to form the small bivalent chromosome at the
center of the group. Without fusing, the two halves are then
immediately separated, the division always taking place more
rapidly than in the case of the larger chromosomes (Fig. I, /).
It is clear to demonstration accordingly, that in Alydus the small
central bivalent does not arise from the large chromosome-nucleolus
of the growth-period, but is formed by the late conjugation of two
separate microchromosomes that have no genetic connection with
that body. The same fact is shown no less clearly in Archimerus
calcarator (which shows eight chromosomes in the first mitosis),
where the m-chromosomes, and the corresponding bivalent, are
of extraordinary minuteness and are so much smaller than the acces-
sory that they could not possibly be confused with the latter (Fig. 3).
Studies on Chromosomes. 517
I believe that in this form, too, the m-chromosomes are fre-
quently recognizable as condensed separate bodies in the growth-
period; but owing to their minute size it is difficult to be sure of
this. In any case, in the period just before the disappearance of
the nuclear membrane they are quite distinct from the "acces-
sory," which is, as in Alydus, immediately recognizable by its
size (Fig. 3, g). From this period, as in Alydus, the latter body
may be traced continuously backward into the growth-period.
The foregoing facts, observed in Alydus and Archimerus are
in close agreement with Montgomery's results on Protenor,
differing only in that the condensation of the m-chromosomes
takes place somewhat later. 1 In Anasa the condensation of these
chromosomes from the diffused condition takes place still later;
and this, combined with the fact that the "accessory" cannot be
certainly distinguished from the other larger chromosomes by its
size, renders the question more difficult of solution than in Alydus,
though I believe the result is equally decisive. In Anasa, as in
Alydus or Archimerus, the small central bivalent of the first
equatorial plate is formed by a very late conjugation of two
separate microchromosomes that only come together as the spindle
forms, precisely as Gross describes in Syromastes. Of this fact
no doubt is left by the study of a large number of preparations
that show every stage of the process, step by step. In the late
prophases, just before the nuclear membrane disappears, the nuclei
invariably show twelve separate, condensed, intensely-staining
chromatin-elements (one more than the number of chromosomes
in the first mitosis) in addition to one or more pale rounded
plasmosomes with which the chromosomes cannot for a moment
be confused. Ten of these are larger bivalents which have the
form of quadripartite tetrads or dumbbell-shaped bodies. The
remaining two are much smaller bodies, irregularly ovoidal or
frequently more or less distinctly bipartite (m, Fig. 2, ^, /); they
may occupy any relative position. As the spindle forms, the
microchromosomes lose their bipartite form, assume an evenly
rounded ovoidal shape, and conjugate at the center of the equa-
torial plate to form a small dyad-shaped bivalent (Fig. 2, -*)
Without fusion the two halves are then immediately drawn apart
'In Alydus pilosulus this author believed the /n-chromosomes, as usual, to be derived from the
large chromosome-nucleolus.
5 i8
Edmund B. Wilson.
FIGURE z.
Anasa tristis. a, Contraction-phase of synaptic period, showing "accessory" (h~) and plasmosome (p);
b, spermatocyte-nucleus, late groarth-period, beginning of the condensation, showing "accessory'' (A)
and the m-chromosomes (m); c, a slightly later stage than the last; d, later stage, immediately before
the final condensation, from a long-extracted preparation; e, /, two sections of one nucleus, show-
ing all of the twelve chromosomes immediately before the disappearance of the nuclear membrane;
g, view of one pole of the late prophase just after disappearance of the nuclear membrane, the m-chro-
mosomes still wide apart; h, early metaphase-group in side view, showing approach of the m-chromo-
somes; *, four chromosomes from the metaphase, conjugation of the m-chromosomes to form the small
central bivalent; j, early anaphase, separation of the m-chromosomes, "accessory'' at the left; k, polar
view of metaphase-group, first division; /, polar view of metaphase-group, second division; m, n,
anaphases of second division, showing division of m-chromosomes and the undivided heterotropic
chromosome; o, p, spermatogonial metaphase-groups drawn as carefully as possible to show sizes of
the chromosomes.
Studies on Chromosomes.
5*9
520 Edmund B. Wilson.
in the initial anaphase, always separating in advance of the
larger chromosome-halves (Fig. 2, /). It is not possible in the
prophases just described to identify the heterotropic chromosome;
but from the analogy of Alydus, Syromastes and Archimerus it
may be assumed with great probability that it is the "odd or
eccentric chromosome which in the metaphase-group lies outside
the principal ring (Fig. 2, k).
During the growth-period, as Paulmier described, the chromo-
somes, with the exception of the single conspicuous chromosome-
nucleolus, remain in a loose, diffused, lightly-staining condition
from the post-synaptic spireme stage until the condensation of the
tetrads begins; and until the end of this period the m-chromo-
somes cannot be distinguished. Throughout this whole period
the chromosome-nucleolus is distinctly visible; and it may at
every period, even in hematoxylin preparations, if long extracted,
be at once distinguished from the true nucleolus or plasmosome
(as is shown in Paulmier's figures), since the former stains intensely
black, the latter pale blue or in double-stained preparations, pale
red or yellow. In the contraction-phase of the synaptic period
it is more or less elongated, ovoidal, or sometimes slightly con-
stricted in the middle (Fig. 2, a). In the late post-synaptic
period, at a time when the other chromosomes are beginning to
shorten and to give rise to the characteristic double cross-figures
and V-figures it is usually more or less elongated, the transverse
constriction is less obvious or disappears from view, and the body
often shows faintly but distinctly a longitudinal split. (Cf. Paul-
mier, Fig. 22.) Slightly later, as the other chromosomes continue
to shorten and thicken, the chromosome-nucleolus also shortens
and thickens, often assuming a spheroidal form in which a central
cavity may sometimes be seen. As the remaining chromosomes
condense to form the tetrads it again alters its shape, often becom-
ing bipartite (Fig. 2, -
A
('
3
* lX M
** ! ^ a- *l /
5
3 4
FIGURE i l
Protenor belfragei. a, Anaphase of second spermatocyte-division; b, c, sister groups, from the
same spindle, polar view, second spermatocyte-division; d, e, /, spermatogonial groups; g, h, groups
from immature ovaries, probably oogonia; i, group from dividing follicle-cell.
d, ^), though the members of each pair may occupy any relative
position. Of these six pairs, one (2, 2) is always much larger than
the others, its members being approximately half the size of the
J A11 the figures are drawn to the same scale. In all, h denotes the heterotropic chromosome, ; the
idiochromosomes (large and small in some cases lettered I and i respectively), m the paired micro-
chromosomes, and i the smallest pair of ordinary chromosomes.
8 Edmund B. Wilson,
heterotropic. A second pair (3, 3) may usually be distinguished
as the next largest, and a third pair (7, 7) as the smallest, though
this is not always obvious. This pair probably correspond
to the " m-chromosomes " of my preceding paper. The remain-
ing three pairs are of nearly equal size, though sometimes they
clearly show a progressively graded series as in Fig. I, ) and the large idiochromosome-pair must
therefore be represented by one of the larger pairs. Fig. 4, n, o,
show the spermatogonial and oogonial groups of E. fissilis, show-
ing the same relations as in E. servus, save that the small pair are
relatively larger.
The above-described species of Euschistus, while agreeing pre-
cisely in the general relations, present individual differences so
marked as to show that even the species of a single genus may be
distinguishable by the chromosome-groups. In this case the
most interesting feature is the series shown in the inequality
of the idiochromosomes, which becomes progressively greater
in the series (i) E. servus, (2) tristigmus, fissilis, (3) ictericus,
(4) variolarius, the inequality in the last case being fully as great
as in Lygaeus. I may again mention the fact that in the opposite
direction the genus Brochymena often shows the idiochromosomes
less unequal than in E. servus; in Mineus they are sometimes
of nearly equal size (Fig. 4, />, m)-
In this case, therefore, alone among all those examined, no
visible differences are shown by the nuclei of the two sexes.
One pair of the chromosomes are, however, different in nature
from the others, as is shown by their different behavior in the
male in the growth-period and in synapsis; and it is quite clear
that the two members of this pair are always assigned to different
spermatozoa. In respect to this chromosome, therefore, the
spermatozoa fall into two classes as truly as the other forms,
though they cannot be distinguished by the eye. It is hardly
necessary to point out how important this case is in giving a firm
basis of comparison with the more usual forms in which, if we can
trust the existing accounts, all of the functional spermatozoa are
exactly alike in appearance, and no sexual differences of the chro-
mosome-groups are apparent.
22 Edmund B. Wilson
E. The Differential Chromosomes in the Synaptic and Growth-
periods
I will now briefly consider a very marked difference between
the sexes in respect to the behavior of the differential chromosomes
during the contraction-phase of synapsis and the succeeding early
growth-period. 1 In the male, as was fully described in my last
paper, both the heterotropic chromosome and the idiochromo-
somes condense early in the growth-period (usually as early as the
contraction-phase of synapsis) to form rounded, condensed,
intensely-staining chromosome-nuclei. In this condition they
persist throughout the whole growth-period of the spermatocyte,
without ever assuming the looser texture and more elongate form
of the other chromosomes. In the earlier part of this period they
are as a rule closely associated with a large pale plasmosome, but
later become separated from it.
In the female no trace of such a chromosome-nucleus can be
found in the contraction-figure of the synaptic period. My best
preparations of this stage are from the ovaries of the larval Anasa,
which show a distinct synaptic zone of oocytes intervening between
the zone of multiplication and the growth-zone; but I have
observed the same condition in the ovaries of recently emerged
adults of Harmostes, Alydus, Euschistus, Coenus and Podisus.
In all these forms the contraction-figure is very similar to that of
the spermatocytes, the chromosomes being in the form of deeply
staining, ragged, and apparently longitudinally split loops that
are crowded into a spheroidal mass toward the center or one side
of the nucleus and surrounded by a large clear space. The nuclei
at this time occasionally show one or two small deeply-staining
nucleolus-like bodies (probably plasmosomes); but these are
much smaller than the chromosome-nuclei of the spermatocytes
at this period, and in many of the nuclei are absent. The contrast
between these nuclei and those of the male at the corresponding
period is so striking as to be at once apparent. In later stages the
chromosomes spread through the nuclear cavity, become looser
in texture and finally give rise to a fine reticular structure. In
J A fuller presentation of observations on these phenomena is reserved for a subsequent paper.
Studies on Chromosomes . 23
these stages a variable number of deeply-staining nucleoli make
their appearance; but their true nature can only be determined
positively when the whole ovarian life of the egg has been followed
and the process of maturation observed. I can, therefore, only
state that no chromosome-nucleolus is present in the contraction
period of synapsis, or in the early growth-period; and even though
it be present in later stages, which I think is very doubtful, a wide
difference between the sexes would still exist in respect to the
earlier period.
F. General Resume
The foregoing results may be given a general formulation as
follows : If n be the unreduced number of chromosomes in the
female, the matured eggs in all cases contain half this number (").
The males are of three types. In the first, one of the chromo-
somes (the heterotropic or "accessory") is without a mate, and
the unreduced number is accordingly one less than that of the
female. Half the spermatozoa possess, and half lack, the hetero-
tropic chromosome, the first class having the same number as the
matured eggs ("), the second class one less (5-1). In the second
type the male has the same number of chromosomes as the female,
but possesses one large and one small idiochromosome while the
female possesses two large ones. In maturation half the spermat-
ozoa receive the small and half the large idiochromosome. The
third type differs from the second in that the idiochromosomes are
of equal size in both sexes, and no visible differences exist between
the two classes of spermatozoa or the somatic groups of the two
sexes. Designating the large and small idiochromosomes as 7
and / respectively, the relations in fertilization and sex-production
are as follows:
TYPE I
(PROTENOR, ANASA, ALYDUS, HARMOSTES)
Egg " + spermatozoon ^ (including heterotropic) = n (female).
Egg 5 + spermatozoon i (heterotropic lacking) = n I (male).
TYPE II
(LYGAEUS, EUSCHISTUS, COENUS, PODISUS)
Egg 5 (including /) + spermatozoon ^ (including 7) = n (including //) (female).
Egg ^ (including 7) + spermatozoon (including ') = n (including /') (male).
24 t Edmund B. Wilson
TYPE III
(NEZARA)
Egg + spermatozoon = n (male or female, including in each case two equal idiochromosomes).
These relations are graphically shown in the following diagram
(Fig. 6) in which the differential chromosomes are black and the
ordinary ones unshaded (only two pairs of the latter shown). For
the sake of simplicity only the final result of synapsis (second
column) and the ensuing process of reduction (third column) are
shown, without regard to variations of detail. The matured eggs
(ov) are represented with a single polar body (the result of the
reduction-division) which is greatly exaggerated in size. The
female-producing and male-producing spermatozoa (sp} are
lettered a and b respectively. It will be evident from an inspection
of this diagram that the second type may readily be derived from
the third, and the first from the second by the reduction (second
type) and final disappearance (first type) of one of the differential
chromosomes. This I believe to represent the actual relations
of the three types.
II. GENERAL.
In recent years evidence has steadily accumulated to strengthen
the view that the general basis of sex-production is given by a
predetermination existing at least as early as the fertilized egg,
but there is a wide divergence of opinion in regard to the condi-
tions preexisting in the gametes prior to their union. 1
The fact that in some organisms (such as Dinophilus, Hyda-
tina or Phylloxera) the unfertilized eggs, sometimes even in the
ovary, are visibly distinguishable as male-producing and female-
producing forms, has led a number of recent writers to deny that
the spermatozoon can play any part in sex-determination. Beard,
for example, asserts that "The male gamete, the spermatozoon,
has and can have absolutely no influence in determining the sex
'The general question of sex-determination, with its literature, has within the past five years
been so ably and thoroughly reviewed by Cue'not, Strasburger, Beard, von Lenhosse'k, O. Schultze
and others, that I shall here limit myself in the main to an analysis of the new observations brought
forward.
Studies on Chromosomes .
OOGONIA FERTILIZATION
AND SYNAPSIS / A
SPERMATOGONIA Gametes Spermatozoa Zygotes
/ Pro tenor
I
/// Nezara
Fio. 6.
26 Edmund B. Wilson
of the offspring" ('02, p. 712); and a similar conclusion, though
less dogmatically stated, is reached in the general reviews of
Lenhossek ('03) and O. Schultze ('03). The opposite view that
the spermatozoon alone is concerned in sex-determination (which
like the preceding one, is of very ancient origin) has, however, been
maintained by some recent writers, for instance, Block (whose
work I know only from Cuenot's review) and McClung, as already
mentioned. 1 On the other hand, both Cuenot ('99) and Stras-
burger ('oo) in their able reviews, have argued that both gametes
may be concerned in sex-determination; and the last named
author urged the view, afterward recognized as probable by
Bateson and developed in detail by Castle ('03), that sex-produc-
tion takes place in accordance with the Mendelian principles of
inheritance.
The observations here brought forward, together with those of
Stevens on Tenebrio, establish the predestination (in a descriptive
sense) of two classes of spermatozoa, equal in number, as male-
producing and female-producing forms. Though indistinguish-
able to the eye in their mature state, these two classes differ visibly
in nuclear constitution at the time of their formation; and since
this occurs in the same order of insects as Phylloxera, where the
eggs are visibly distinguishable (by their size) as male-producing
and female-producing forms, it is evident that a substantial basis
now exists for the views expressed by Cuenot and Strasburger, and
for the Mendelian interpretation of sex-production worked out
by Castle. Whether in the Hemiptera that form the subject of
this paper the eggs are, like the spermatozoa, predestined as male-
producing and female-producing forms can at present be a matter
of inference only. I have not been able to distinguish such classes
by their size, and the data show, almost with certainty, that if
they exist they do not exhibit any visible nuclear differences
like those present in the spermatozoa. But this gives no ground
for denying their existence. No visible nuclear dimorphism of
J "By exclusion then, it would seem that the determination of this difference (the sexual one) is
reposed in the male element" (McClung, '02, p. 78). McClung nevertheless maintained the exist-
ence of a selective power on the part of the egg such that " the condition of the ovum determines
which sort of spermatozoon shall be allowed entrance into the egg substance" (op. cit., p. 76).
Studies on Chromosomes 27
the spermatozoa exists in Nezara, yet this condition is con-
nected, by an almost continuous series of intermediate forms,
with one in which a conspicuous difference of nuclear con-
stitution is to be seen. It seems hardly open to doubt that
sex-production conforms to the same essential type throughout
this series. At least a possibility is thus established that in
organisms generally both eggs and spermatozoa may be pre-
destined as male-producing and female-producing forms, whether
they are visibly different or not. In any case, it is evident
that in the Hemiptera the chromosome-combination characteristic
of each sex is established by union of the gametes and is a result
of fertilization by one or the other of the two forms of spermatozoa.
Sex must therefore already be predetermined in the fertilized egg,
and it is difficult to conceive how it could subsequently be altered
in these animals by conditions external to the egg or embryo.
Since the idiochromosomes or heterotropic chromosomes form the
distinctive differential between the nuclei of the two sexes, it is
obvious that these chromosomes are definitely coordinated with
the sexual characters. We must therefore critically inquire into
the causal relation between sex-production and the chromosomes,
of which this coordination is an expression.
That sex-production may be interpreted as the result of a
Mendelian segregation, transmission and dominance of the sexual
characters has been shown by Castle ('03). The history of the
differential chromosomes in synapsis and reduction evidently
affords a concrete basis for such an interpretation in the terms of
the Sutton-Boveri chromosome-theory. Analysis of the facts now
known will, however, show even more clearly than the more
general considerations adduced by Castle, that this interpretation
is only admissible under the assumption that a selective fertiliza-
tion occurs, such that eggs containing the female-determinant are
fertilized only by spermatozoa containing the male-determinant
and vice versa. Until I had read Cuenot's recent interesting
paper ('05) on the breeds of mice and their combinations, the
necessity for making this assumption seemed to me an almost
fatal difficulty in the way of the interpretation, but if Cuenot's
conclusions be well founded the a priori objections to such a
28 Edmund B. Wilson
selective fertilization are in large measure set aside. I therefore
think that the possibility of a Mendelian interpretation of sex-
production should be carefully examined, though as will be shown,
an alternative interpretation is possible.
I. In such an examination the distinction between sex-deter-
mination and sex-inheritance should be clearly drawn; 1 for it is
well known that each sex may contain factors capable of pro-
ducing the characters of the opposite sex, and it may well be that
the patency or latency of the sexual characters is determined by
factors quite distinct from those concerned with their transmission
from parent to offspring. For the purpose of analysis it will,
however, be convenient to speak of the idiochromosomes or their
homologues as "sex-determinants," this term being understood
to mean that these chromosomes are the bearers of the male and
female qualities (or the factors essential to the production of these
qualities) respectively. They may also be designated (whenever
it is desirable to avoid circumlocution) as .sex-chromosomes or
"gonochromosomes." As a basis of discussion the Mendelian
interpretation may be taken to postulate, further, that the two
sex-chromosomes, which couple in synapsis and are subsequently
disjoined by the reducing division, are respectively male-determi-
nants and female-determinants in the sense just indicated. The
most convenient approach to the question is offered by the hetero-
tropic chromosome, since its unpaired condition in one sex renders
its mode of transmission more clearly obvious than that of the
idiochromosomes. The facts (especially as observed in Protenor)
clearly prove that this chromosome alternates between the sexes
in successive generations, passing from the male to the female in
the production of females, and from the female to the male in the
production of males (Fig. 6). The important bearing of this
on both sex-inheritance and sex-determination will appear beyond.
Since the heterotropic chromosome is without a fellow in the
male it must, if it be a sex-determinant at all, be the male-determi-
nant, which exerts its effect uninfluenced by association with a
female-determinant. But since the spermatozoa that contain
C/. Watase, '92.
Studies on Chromosomes 29
this chromosome produce only females, it must be assumed that
the maternal mate or fellow, with which it becomes associated on
entering the egg, is a dominant female-determinant. Further,
since males result from fertilization by spermatozoa that do not
contain the heterotropic chromosome, the latter must in male-
producing eggs be derived from the egg-nucleus (cf. the diagram,
Fig. 6). The general interpretation, therefore, must include the
assumption that there are two kinds of eggs (presumably in
approximately equal numbers) that contain respectively the male-
and the female-determinant, 1 and that the former are fertilized only
by spermatozoa that lack the heterotropic chromosome (/. e., the
male determinant) and vice versa, 2 giving the combinations (m)f
(female) and m (male). Such a selective fertilization is there-
fore a sine qua non of the assumption that the heterotropic chro-
mosome is a specific sex-determinant.
A nearly similar, though somewhat more complex, result follows
in the case of the idiochromosomes. In respect to sex-production
the large idiochromosome is identical with the heterotropic chro-
mosome, and the morphological evidence is nearly or quite
decisive that the heterotropic chromosome is actually a large
idiochromosome, the smaller mate of which has disappeared.
The small idiochromosome may therefore be regarded as a
disappearing, or even vestigial, female-determinant that is recessive
to its larger fellow (the male-determinant); and its reduction in
size may plausibly be regarded as an atrophy resulting from its
invariably recessive nature (this chromosome being strictly con-
fined to the male). Precisely as in case of the heterotropic
chromosome, the large idiochromosome of the male (male-
determinant) must be derived in fertilization from the egg-nucleus
(Fig. 6); and, as before, it must be assumed that eggs that contain
this chromosome are fertilized only by spermatozoa that contain
the small idiochromosome, those that contain the female-determi-
'This would follow from the coupling of the two sex-chromosomes in synapsis to form the bivalent
(m)j, and its division in such a way as to leave in the egg either the male- or the female-determinant
indifferently.
Otherwise the combinations mm or / might result, which is contrary to observation, since the sex-
chromosomes are in this type never paired in the male or unpaired in the female.
30 Edmund B. Wilson
nant only by spermatozoa containing the large idiochromosome.
In this type, accordingly, it is clear that the large idiochromosome
(like the heterotropic chromosome to which it corresponds) passes
alternately from one sex to the other, while the small one never
enters the female; and this would remain true even did selective fer-
tilization not occur (Fig. 6). The same interpretation may finally
be extended to Nezara, where the idiochromosomes are of equal size
in both sexes, the relations of dominance being the same as before.
The two vital points in this result are first, the assumption of
selective fertilization, and second the relations of dominance and
recession in the two sexes. As regards the first point, until the
appearance of Cuenot's paper, referred to above, almost no
definite evidence had been produced of an infertility between
particular classes of gametes in the same species; though it has
long been known that many plants are in a greater or less degree
infertile to their own pollen, and an analogous fact has been more
recently demonstrated in Ciona by Castle ('96) and Morgan ('04).
Correns ('02), in his study of hybrid maize, was led to suggest
that in this case there might be a somewhat diminished fertility
between the gametes bearing the recessive character (thus account-
ing for a relative deficiency of extracted recessives in the second
generation of crosses, F 2 ). In studying the breeds of mice
Cuenot has found it impossible to obtain pure or homozygous yellow
forms. Yellow mice are invariably heterozygotes (the yellow
being dominant over gray, black or brown) and when crossed
with a pure race of a different color (e. g., gray) give the typical
Mendelian result, yellow and gray offspring appearing in equal
numbers. This proves that a complete Mendelian disjunction
of the yellow and gray determinants takes place in maturation.
When yellow mice of known constitution (e. g., Y(G)) are paired
with like forms, the first offspring include pure gray forms (ex-
tracted recessives) slightly in excess of the normal ratio of 25 per
cent., and yellow forms; but contrary to the Mendelian expec-
tation the latter, when paired with one another, never give pure
dominants (YY), but again produce pure grays (GG) and
heterozygous yellows (Y(G)). Cuenot therefore concludes that
although complete segregation of both the gray and yellow
Studies on Chromosomes 3 1
characters takes place in the gamete-formation, and the resulting
yellow-bearing gametes unite freely with those bearing the reces-
sive color, they do not unite with each other: "Ceux-ci (the
yellow heterozygotes) forment bien des gametes de valeur CJ ou
AJ, mais ces gametes ne peuvent pas s'unir les uns aux autres pour
donner des zygotes ayant les formules CJCJ, AJAJ ou CJAJ;
par autre, ils s'unissent facilement a tous les autres gametes que
j'ai essay es pour former avec eux des heterozygotes mono- ou
dihybrides" (op. cit., p. cxxx). This conclusion is sustained by
the fact that the combination Y(G) x Y(G) (CYCG x CYCG in
Cuenot's terminology) produces a relative deficiency of yellows
in the offspring, as is to be expected. 1 In pairing Y(G) with
Y(G), accordingly, the Y-bearing spermatozoa unite only with
the G-bearing eggs, and vice versa, which is exactly analogous
to the selective fertilization assumed in case of the sex-bearing
gametes. Perhaps it may be possible to find a different expla-
nation of the facts; but if Cuenot's interpretation be well-founded
the case goes far to remove the scepticism which I think one must
otherwise feel in regard to a selective fertilization of the gametes
in sex-production.
An examination of the question of dominance involved in the
Mendelian interpretation leads to some interesting conclusions.
In forms possessing unequal idiochromosomes the sexual formulas
would be for the female (m) f and for the male m (/) (/ being the
small idiochromosome). Applying the same interpretation to
Nezara, where the idiochromosomes are of equal size, the corre-
sponding formulas are (m) f and m (/), giving the gametes (m),
/, m and (/). Assuming likewise a selective fertilization the facts
would be:
EGGS SPERMATOZOA
() + (/) = ( m ) (/) producing a male, m(f).
f + m = mf, producing a female (m) /.
rfhe deficiency, though constant, is very slight. Cuenot himself seems to consider this a difficulty,
but I believe a very simple explanation may be given. With equal numbers of the gametes of both sexes
the ratio of yellows to grays should be two to one, instead of three to one as in the typical Mendelian case
(since the class YY is missing). If, however, the spermatozoa be in large excess, as they undoubtedly
are, all or nearly all the Y-bearing eggs will be fertilized by G-bearing spermatozoa, and vice vena, thus
bringing the ratio of yellows (Y(G)) to grays (GG) more or less nearly up to three to one.
32 Edmund B. Wilson
Now it is clear that if the relations of the chromosomes to sex-
production be the same here as in the second type, the chromo-
some m must alternate in successive generations between the male
and the female (like the large idiochromosome or the heterotrqpic
chromosome to which it corresponds), and hence also shows an
alternation of dominance, being dominant in the former sex and
recessive in the latter. If, therefore, dominance and recession
be inherent in the chromosomes, there must be such a relation
between them that m is always dominant to the chromosome (/)
of the male, and always recessive to the chromosome / of the
female, and that the latter two chromosomes (/ and (/)) are never
interchanged between the sexes. This last assumption is not so
improbable as it may at first sight appear; for in the second type
it is certain, as already pointed out, that the small idiochromo-
some ((/) under the general assumption) never enters the female,
while the large idiochromosome, m, like the heterotropic, alter-
nates between the two sexes in successive generations.
A strict Mendelian interpretation of sex-production may unques-
tionably, I think, be constructed upon the foregoing assumptions.
But an interesting suggestion for a somewhat modified Mendelian
interpretation is given by the possibility that the dominance of
the sex-chromosomes is determined by extrinsic factors, namely,
by conditions in the protoplasm of the zygote. If this were the
case it is evident that the idiochromosomes could not be considered
as sex-determinants in the strict sense of the word. The determi-
nation of sex would in this case be due to factors preexisting in
one or both of the gametes, irrespective of the sex-chromosomes,
and the latter could only be considered as a means by which the
sex-characters are transmitted or inherited. The possibility is
here clearly offered that either or both forms of gametes may be
predetermined as males or females (or at least male-producing
and female-producing) prior to fertilization and irrespective of the
chromosomes; and thus an interpretation of the ordinary forms
of gametes would be reached in harmony with such cases as
Dinophilus and other forms in which male-producing and female-
producing eggs are distinguishable in size prior to fertilization.
Such an interpretation would further be perfectly consistent with
Studies on Chromosomes 33
the modification of sex-production in some cases by external condi-
tions, and with the production of both males and females in
parthenogenesis (though this may be otherwise explicable); and
it might also give the explanation of selective fertilization.
II. It has not been my intention to advocate the foregoing
interpretation, but only to set forth as clearly as possible, the as-
sumptions that it involves. It is nevertheless my opinion that the
analysis places no insuperable obstacles in its way, and that,
however dominance be determined, the Mendelian interpretation
may in fact give the true solution of the problem. I have, how-
ever, endeavored to seek for a different interpretation that may
escape the necessity for assuming a selective fertilization; and
although I have to offer nothing more than suggestions, some of
which undoubtedly encounter serious difficulties, I shall make
them in the hope that they may afford some clue to further inquiry.
Some of these suggestions are equally applicable to the Mendelian
interpretation considered above, but for the purpose of discussion
this interpretation may for the time be laid aside.
It seems possible that the differential chromosomes may per-
form a definite and special function in sex-production without being
in themselves specifically male-determining and female-determin-
ing or even qualitatively different save in the degree of their special
activity (whatever be its nature). This suggestion is given by
the fact that the presence of one heterotropic chromosome or
large idiochromosome is associated with the production of a
male, while if two such chromosomes are present a female is
produced. This very obviously suggests that the same kind of
activity that produces a male will if reinforced or intensified
produce a female; and with this would accord the production
of males from unfertilized eggs, and females from fertilized ones,
in the case of the bee. In these cases the decisive factor may be
a merely quantitative difference of chromatin between the two
sexes. But it is obvious that such a difference cannot give the
basis for a general explanation, since in Nezara, and presumably
in many other organisms, both the number of chromosomes and
the quantity of chromatin is the same in both sexes. And yet
34 Edmund B. Wilson
the existence of a quantitative difference in some cases raises the
question whether it is not the result or expression of some more
deeply lying nuclear difference which may still be present in those
cases where no quantitative difference exists. I find it altogether
incredible that two animals as nearly related as Nezara and
Euschistus should differ fundamentally in the relation of the
chromosomes to sex-production; and if there is any reason to
conclude that sex-determination is effected by the idiochromo-
somes (or by the combination of which they form a part) in the
case where they are visibly different, I cannot avoid the belief
that this conclusion applies with equal reason to the case in which
they appear to the eye alike in all the spermatozoa. It therefore
seems to me an admissible hypothesis that a physiological or
functional factor may be present that differentiates the spermat-
ozoa into male-producing and female-producing forms irrespec-
tive of the size of the differential chromosomes; and further,
that the morphological difference that has arisen in some forms
may have been a consequence of such an antecedent functional
difference. If we could assume for instance that the differential
chromosome-pair in the male includes a more active and a less
active member (the latter having in many cases become reduced
in size or even having entirely disappeared) the suggestion might
be greatly extended in application. Under this assumption the
facts might receive a general formulation in the statement that
the association of two more active chromosomes of this class
produces a female, while the association of a more active and a
less active one (or the absence of the latter, as in case of the hetero-
tropic chromosome) produces a male. Reduction of the less
active member to form a small idiochromosome would introduce
a quantitative difference of chromatin as well as a qualitative one.
Its complete disappearance in the male, leaving only the active
member as the heterotropic chromosome, would reduce the
difference to a merely quantitative one. The assumption of
such a physiological difference is admittedly a purely specula-
tive construction, and may seem a priori very improbable. But
from the a priori point of view it would seem equally improbable
that a morphological dimorphism of the spermatozoa, affecting
Studies on Chromosomes 35
only one pair of the chromosomes, should have arisen; yet this
is an observed fact. I therefore think the suggestion is worthy
of serious consideration. If it could be adopted the necessity
of selective fertilization would be avoided, for the observed results
would follow from the fertilization of any egg by any spermatozoon.
But even if in accordance with fact the suggestion is still
obviously incapable of direct application to cases in which sex
is determined independently of fertilization for instance, sex-
production in parthenogenetic development or in hermaphrodites,
and in forms (such as Dinophilus) where male-producing and
female-producing eggs are distinguishable in size before fertiliza-
tion. It is possible that these cases may be explicable (under either
general interpretation) as a result of some forms of differential
distribution of the chromosomes occurring at the time of the for-
mation of the polar bodies (parthenogenesis) or at some earlier
period in the cell-lineage of the germ-cells; and this possibility
should of course be tested by a close cytological study of the facts.
On the other hand, there is nothing in the facts to negative the
assumption that in some cases the chromosome-combination,
established at fertilization, may be in something like a balanced
state that is capable of modification by conditions external to the
nucleus (as already suggested in the case of dominance).
Boveri's interesting observations on the dispermic eggs of
Ascaris ('04) have given direct evidence that the chromosomes
react to their cytoplasmic surroundings; and the same fact is
even more clearly shown by the difference of behavior of the
differential chromosomes in the two sexes of Hemiptera during the
synaptic and growth-periods. Hence, even though a preestab-
lished basis of sex-determination be given in such a physiological
dimorphism of the spermatozoa as I have suggested, the sex of
the fertilized eggs may in many cases be only a matter of greater
or less predisposition and not an immutable predetermination.
The nuclei, and hence the primordial germ-cells, may in such
cases be in a state of approximate equilibrium, and still retain the
power of response to varying conditions in the cellular environ-
ment. The production of eggs or spermatozoa in hermaphro-
dites may thus be explicable as a result of greater or less nuclear
36 Edmund B, Wilson
activity in the two cases, incited by intra-cellular conditions that
are external to the chromosome-groups; and a similar explana-
tion may apply to the related case of the formation of visibly
different female-producing and male-producing eggs in the same
organism.
It would not, I think, be profitable to speculate further in regard
to these special cases, but I have wished to indicate that a hypo-
thesis of sex-production which recognizes in some cases a fixed
predetermination in the chromosome-groups of the fertilized egg
is not inconsistent with the control of sex-production in other
cases by conditions external to the nucleus. The constant
chromosomal differences of the sexes existing in many Hemiptera,
therefore, by no means preclude experiments on the modification
or control of sex-production.
I have intentionally excluded from the foregoing suggestions
any discussion of the specific nature of the activities of the differen-
tial chromosomes, since we are almost wholly ignorant of the
functions of chromosomes in general. But although we here
enter upon still more debatable ground, I think we should not
hesitate to consider such possibilities in this direction as the facts
may suggest.
One of the principal, or at least most obvious, differences
between the germ-cells of the two sexes is their great contrast in
constructive activity, evinced by the enormous growth of the
primary oocyte as compared with the primary spermatocyte.
This growth of the oocyte involves the production of a mass of
protoplasm (including under this term the yolk or metaplasm as
well as the active protoplasm) thousands of times the bulk of the
spermatocyte; and although the latter also increases noticeably
in size during the growth-period, the accumulation of proto-
plasm is almost insignificant as compared with that which takes
place in the female. Now, as described above, the idiochromo-
somes and heterotropic chromosome remain during this period in
the male in a relatively passive condition as compared with the
other chromosomes, while this is not the case in the female. The
thought cannot be avoided that there is a definite causal connec-
o
tion between the greater activity of these chromosomes in the
Studies on Chromosomes 37
oocytes and the great preponderance of constructive activity in
these cells; and it is especially this coincidence that leads me to
the general surmise that one of the important physiological
differences (I do not say the only one), between the chromosome-
groups of the two sexes, may be one of constructive activity.
I have elsewhere (The Cell, Chapter VII) reviewed at some
length the evidence pointing toward the conclusion that the
nucleus (more specifically, the chromatin) is especially concerned
with the constructive processes of cell metabolism; and while I no
longer hold the view that the nucleus can be considered as the
actual formative center of the cell, it still seems to me very
probable that the formative processes are directly or indirectly
under its control, as has been advocated by many students of
cell-physiology. If this view be well-founded, the facts observed
in Hemiptera give a very definite and concrete basis for assuming
a greater constructive activity in the cells of the female generally,
which reaches a climax in the growth-period of the oocyte. 1 It
seems possible that some of the specific differentiations that take
place in the later history of the germ-cells may be directly trace-
able to the primary difference in the growth-process. It is well
known that the young oocytes and spermatocytes show a very
close similarity, not only in size but also in many details of struc-
ture. The enormous accumulation of cytoplasm in the oocyte
as compared with the spermatocyte leaves the latter with a great
relative excess of the kinoplasmic or archoplasmic material in which
the most characteristic differentiations of the spermatozoa such as
the acrosome, middle-piece, axial filament and tail-envelopes
take their origin. Perhaps a direct causal relation here exists.
'This suggestion recalls the theory developed by Geddes and Thomson, in their well known work on
the "Evolution of Sex," that " the female is the outcome and expression of relatively preponderant anabo-
lism, and the male of relatively preponderant katabolism" (pp. cit., revised ed., 1901, p. 140). As de-
deloped by these authors, this theory has always seemed to me to have too vague and general a character
to have much practical value, though it expresses a certain physiological contrast between the sexes that
undoubtedly exists. My suggestion is only remotely connected with that theory, since it refers the differ-
entiation of the sexes to a functional difference that preexists in the cells of the male, and involves no
contrasted processes of anabolism and katabolism. Nevertheless, the observations here brought forward
may harmonize with that side of the theory which lays stress on the preponderant constructive activity of
the female cells.
38 Edmund B. Wilson
III. Though I have found it convenient to consider the two
foregoing interpretations separately, they evidently have many
points of agreement, and perhaps may be reduced to a common
basis. Both assign to the differential chromosomes a specific
function in sex-production, both recognize the possibility of a
determination of sex (as opposed to its transmission), by con-
ditions external to the chromosome-groups, and both assume,
in one sex, a specific difference in the sex-chromosomes, followed
by a Mendelian disjunction in the formation of the gametes.
The essential point in which the second interpretation diverges
from the first is that the sex-chromosomes are not conceived as
bearing the male or female qualities respectively but as differing
only in the degree of their activity, and this difference is assumed
to exist in the male only (owing to the relation of fertilization
to sex-production). It must be admitted that each interpreta-
tion involves a considerable element of pure conjecture, and that
each includes assumptions which without additional data must
be considered as serious difficulties. The principal one involved
in the first interpretation is the assumption of selective fertiliza-
tion; but if this assumption be granted I believe that it may give
an adequate solution of the problem of sex-production in the sexual
reproduction of dioecious organisms. The second interpreta-
tion avoids this difficulty; it may explain the primary difference
between the gametes of the two sexes, the latency of female
characters in the male, and the development of such secondary
female characters as may be regarded as an exaggeration or inten-
sification of corresponding characters in the male. It seems con-
spicuously to fail to explain the reverse case of characters that
are more highly developed in the male; and to many this will
doubtless appear a fatal difficulty. But we are still ignorant of
the action and reaction of the chromosomes on the cytoplasm and
on one another, and have but a vague speculative notion of the
relations that determine patency and latency in development.
Additional data will therefore be required, I think, to show
whether the difficulty in question is a fatal one, and in what meas-
ure either of the two general interpretations that have been con-
sidered may approach the truth. The positive result of the
Studies on Chromosomes 39
observations of Stevens and myself is to demonstrate the existence
of a constant and definite correlation between the chromosomes
and the sexual characters, which is visibly expressed in the relations
of a single pair of chromosomes. These relations unquestionably
afford a concrete basis for an interpretation of sex-production
that assumes a Mendelian segregation and transmission of the
sex-characters and to this extent they accord with the general
assumption of Castle. The validity of both this and the alterna-
tive interpretation suggested must be tested by further inquiry.
Zoological Laboratory of Columbia University,
December 8, 1905.
WORKS CITED
BEARD, JOHN, '02. The Determination of Sex in Animal Development. Fischer,
Jena.
BOVERI, T. H., '04. Protoplasmadifferenzierung als auslosender Faktor fur Kern-
verschiedenheit. Sitzungsber. der Physikal.-med. Ges. Wiirz-
burg, 1904.
CASTLE, W. E., '96. The Early Embryology of Ciona intestinalis. Bull. Mus.
Comp. Zool., xxvii.
'03. The Heredity of Sex. Ibid., xl, 4.
CORRENS, C., '02. Scheinbare Ausnahme von der Mendel'schen Spaltungsregel
fur Bastarde. Ber. d. deutschen Bot. Ges., xx.
CUENOT, L., '99. Sur la determination du sexe chez les animaux. Bull. Sci. de
la France et de la Belgique, xxxii, v, I.
'05. Les races pures et leurs combinaisons chez les souris. Arch. Zool.
Exp. et Gen. (4), iii, Notes et Revue, No. 7.
GROSS, J., '04. Die Spermatogenese von Syromastes marginatus. Zool. Jahrb.,
Anat. und Ontog., xx, 3.
HENKING, H., '91. Ueber Spermatogenese und deren Beziehung zur Eientwick-
lung bei Pyrrochoris apterus. Zeitschr. Wiss. Zool., li.
LENHOSSEK, M. v., '03. Das Problem der geschlechtsbestimmenden Ursachen.
Fischer, Jena.
McCLUNG, C. E., '02. The Accessory Chromosome. Sex-determinant? Biol.
Bull., iii, I, 2.
MONTGOMERY, T. H., '01. A Study of the Germ Cells of Metazoa. Trans.
Am. Phil. Soc., xx.
'04. Some Observations and Considerations on the Maturation Phenom-
ena of the Germ-cells. Biol. Bull., vi, 3.
40 Edmund B. Wilson
MORGAN, T. H., '04. Self-fertilization Induced by Artificial Means. Jour.
Exp. Zool., i, I.
SCHULTZE, O., '03. Zur Frage von den geschlechtsbildenden Ursachen. Arch,
mik. Anat., Ixiii.
STEVENS, N. M., '05. Studies in Spermatogenesis with especial Reference to the
"Accessory Chromosome." Publication No. 36, Carnegie Insti-
tution of Washington, Sept., 1905.
STRASBURGER, E., 1900. Versuche mit diocischen Pflanzen in Riicksicht auf
Geschlechtsverteilung. Biol. Centralbl., xx, 20-24.
SUTTON, W. S., '02. On the Morphology of the Chromosome-group in Brachy-
stola magna. Biol. Bull., iv, I.
WALLACE, L. B., '05. The Spermatogenesis of the Spider. Biol. Bull., viii, 3.
WATASE, S., '92. On the Phenomena of Sex-differentiation. Journ. of Mor-
phology, vi, 3.
WILSON, E. B., '05, I. Studies on Chromosomes. I. The Behavior of the Idio-
chromosomes in Hemiptera. Journ. Exp. Zool., ii, 3.
'05, 2. The Chromosomes in Relation to the Determination of Sex in
Insects. Science, xxii, 564, Oct. 20, 1905.
'05, 2. Studies on Chromosomes. II. The Paired Microchromosomes,
Idiochromosomes and Heterotropic Chromosomes in Hemiptera.
Journ. Exp. Zool., ii, 4.
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STUDIES ON CHROMOSOMES
IV THE "ACCESSORY" CHROMOSOME IN SYROMASTES AND
PYRROCHORIS WITH A COMPARATIVE REVIEW OF
THE TYPES OF SEXUAL DIFFERENCES OF
THE CHROMOSOME GROUPS
By
EDMUND B. WILSON
RETURN TO
DIVISION OF GENETICS
HILGARD HALL
REPRINTED FROM
THE JOURNAL OF EXPERIMENTAL ZOOLOGY
Volume VI No. i
JANUARY, 1909
BALTIMORE, MD., U. S. A.
WILLIAMS & WILKINS COMPANY
STUDIES ON CHROMOSOMES
IV THE "ACCESSORY" CHROMOSOME IN SYROMASTES AND
PYRROCHORIS WITH A COMPARATIVE REVIEW OF THE
TYPES OF SEXUAL DIFFERENCES OF THE CHROMOSOME
GROUPS 1
BY
EDMUND B. WILSON
WITH Two PLATES AND Two FIGURES IN THE TEXT
Since the unpaired idiochromosome ("accessory chromosome")
was first discovered by Henking ('91) in Pyrrochoris apterus L. this
species has been reexamined by only one observer, Dr. J. Gross
('07), with results that are in substantial agreement with those that
pe had reached in an earlier investigation ('04) on the coreid
species Syromastes marginatus L. In both cases his conclusions
hre in conflict with the view advanced by McClung ('02), and first
1 Terminology. With the advance of our knowledge of the chromosomes that form the distinctive
differential between the chromosome groups of the two sexes, and between the male producing and the
female producing spermatozoa, it becomes increasingly difficult to find a common name that will apply
equally to their various modifications. Terms such as the "accessory," "odd" or "heterotropic" chro-
mosome, or "monosome," that are based on the condition of these chromosomes in the male only, are
misleading or inappropriate; and some of them are in certain cases inapplicable, even in the male
e. g., in Syromastes, where the "accessory" chromosome is not univalent but bivalent. Such terms as
"heterochromosome" or "allosome" (Montgomery) seem to me unsatisfactory, since they designate the
m-chromosomes as well as the differential chromosomes, though these are obviously of quite different
nature. Since it has now become evident that a univalent "accessory" chromosome in the male is
exactly equivalent to what I have called the "large idiochromosome" in other forms, I think these
chromosomes should be designated by the same name, and one that will apply equally to both sexes.
While there are some objections to the word "idiochromosome" as a general term for this purpose I
am not able to suggest a better one; and since it has already teen thus employed by some writers, I
shall use it hereafter in a broader sense than that in which I first proposed it, to designate the differential
chromosomes in general, whether they are paired or unpaired in the male, and whether one or more
pairs are present. A univalent or odd idiochromosome in the male will be called the unpaired idiochro-
mosome (or simply the idiochromosome), while the word "heterotropic" may perhaps conveniently
be used as descriptive of its passage without division to one pole in one of the maturation divisions. In
Syromastes, as will appear, the "accessory" or heterotropic chromosome represents a pair of idiochro-
mosomes; while in Galgulus there are several pairs of these chromosomes.
THE JOURNAL or EXPERIMENTAL ZOOLOGY, VOL. vi, NO. I.
JO Edmund B. Wilson
shown to be correct in principle by the work of Stevens and my-
self, that half the spermatozoa are male producing and half female
producing. This view rests on the following facts. When the
male somatic chromosome groups contain an odd number, includ-
ing an odd or unpaired idiochromosome (as in Anasa, Alydus, or
Protenor) the female groups have one more chromosome, being
duplicates of the male groups with the addition of another chro-
mosome like the unpaired one of the male. When the male groups
contain an even number, including a large and a small idiochro-
mosome (as in Lygaeus, Coenus or Tenebrio) the female groups
contain the same number, but include two large idiochromosomes
in place of a large and a small one. In the first type half the
spermatozoa receive the odd idiochromosome while half do
not, the former accordingly containing one chromosome more
than the latter. In the second type all the spermatozoa receive
the same number of chromosomes, but half receive the large
idiochromosome and half the small. It follows from these rela-
tions that eggs fertilized by spermatozoa containing the odd chro-
mosome, or its homologue the large idiochromosome, must pro-
duce females, those fertilized by the other spermatozoa males.
These cytological results, first reached by Stevens ('05) in Tene-
brio (which has a pair of unequal idiochromosomes in the male)
and myself ('o5b, 'o5c, '06) in Anasa, Protenor, Alydus and
Harmostes (which have an unpaired idiochromosome in the male)
and in Lygaeus, Coenus, Podisus and Euschistus (which agree
essentially with Tenebrio), have since been confirmed in a con-
siderable number of species and extended to several other orders
of insects. 2 They have recently received indirectly a striking
experimental confirmation in the important work of Correns ('07),
which proves that in the dioecious flowering plant, Bryonia dioica,
the pollen grains are likewise male determining and female deter-
mining in equal numbers.
Gross's conclusion in the case of Syromastes and Pyrrochoris
is opposed to all these results in that only one of the two forms of
spermatozoa is supposed to be functional (those containing the
a See the tabular review in the sequel.
Studies on Chromosomes JI
"accessory" chromosome) the others being regarded as in a certain
sense comparable to polar bodies (as was also supposed by Wallace
('05). 3 This result was based mainly on the numerical relations,
and especially on the belief that in both these forms the number of
chromosomes is an even one and the same in both sexes twenty-
two in Syromastes, twenty-four in Pyrrochoris. Since the com-
plete reduced number (eleven and twelve in the two respective
cases) is present only in those spermatozoa that contain the
"accessory" chromosome, Gross argues that this class alone can
be concerned in fertilization, as follows :
Syromastes Egg n + spermatozoon n = 22(6" or 9)
Pyrrochoris ...Egg 12 + spermatozoon 12 = 24(d 1 or 9)
whereas in Anasa or Protenor the relations are:
Anasa Egg u + spermatozoon 10 = 21 (c?)
Egg ii + spermatozoon n = 22 ( 9)
Protenor Egg 7 + spermatozoon 6 = 13 (c?)
Egg 7 + spermatozoon 7 = 14(9)
In the hope of clearing up this perplexing contradiction I
endeavored to procure material for a reexamination of the two
forms in question, and through the great kindness of Professor
Boveri, to whom my best thanks are due, was fortunate enough to
obtain an abundant supply of both, though unluckily it includes
no female material. 4 As far as the relations can be worked out on
the male alone they give, I believe, the solution of the puzzle and
bring the two species in question into line with the general princi-
ple that has been established for other forms. This is evidently
true of Pyrrochoris. i Syromastes, however, constitutes a new
3 At first thought this seems to be in harmony with the remarkable discovery of Meves ('03, '07) that
in the male honey bee actual polar bodies are formed which produce abortive spermatids. Butobviously
the two cases are not parallel, for in the bee the fertilized eggs produce only females; and this finds a
natural explanation, in accordance with the general conclusions of McClung, Stevens and myself, in the
assumption that it is the male producing class that degenerate as polar bodies.
4 The material, fixed in Flemming's fluid and in Bouin's picro-acetic-formol mixture, is of excellent
quality and gave preparations of perfect clearness. The Flemming material is on the whole the best.
For single stains Zwaardemaker's safranin and iron haematoxylin were employed (the latter especially
for photographs). Various double stains were also used. One of the best, which I can strongly recom-
mend to other workers in this field, is the combination of safranin and lichtgriin, which gives prepara-
tions of admirable clearness and is also easy to use and certain in its results.
72 Edmund B. Wilson
type that is not yet known to be exactly paralleled in other
forms; though, as will appear, the genus Galgulus presents a
somewhat analogous case. It does not seem to have occurred to
Dr. Gross (as it did not to me until I had carefully studied both
forms) that Syromastes and Pyrrochoris might be of different
type, but such is evidently the case. I shall endeavor to show that
Pyrrochoris is of quite orthodox type, having an odd somatic
number in the male and a typical unpaired idiochromosome.
Since I am compelled to differ with Dr. Gross in regard to this
species, I am glad to admit that the doubts I formerly expressed
as to his account of the spermatogonial number in Syromastes,
were unfounded. In regard to the female number, on the other
hand, I believe he was misled by a wrong theoretic expectation
(as he evidently was in case of the male Pyrrochoris) , though it
is possible that his determination of the apparent number was
also correct, as indicated beyond.
SYROMASTES MARGINATUS L.
Gross's account of this form was as follows: The somatic
groups in both sexes are stated to show twenty-two chromosomes.
The " accessory" chromosome arises by the synapsis of two
spermatogonial chromosomes, and is therefore a bivalent. It
divides equationally in the first spermatocyte division but fails to
divide in the second, passing bodily to one pole in advance of the
other chromosomes without even entering the equatorial plate.
All of the spermatid-nuclei thus receive ten chromosomes and
half of them in addition the "accessory." These are the essen-
tial conclusions; but they are complicated by the following singular
view of the relations between the "accessory" and the micro-
chromosomes or "m-chromosomes." The chromosome nucleolus
of the growth period is supposed not to give rise (as it does in
Pyrrochoris and other forms) to the heterotropic or "accessory"
chromosome of the spermatocyte divisions, but to the m-chromo-
some bivalent the same view as the earlier one of Paulmier
('99) which has since been shown to be erroneous (Wilson '05 c).
But, on the other hand it is believed to arise, not from the
Studies on Chromosomes 73
ra-chromosomes of the spermatogonia, but from two larger chro-
mosomes, while the spermatogonial ra-chromosomes are supposed
to be converted into the "accessory" (!). I will not enter upon the
very ingenious, if somewhat fantastic, conclusions that are based
on these results, for, as I shall attempt to show, the results them-
selves cannot be sustained in some important particulars. But
apart from this I am glad to be able to give the most positive con-
firmation of Gross's interesting discovery in regard to the numer-
ical relations in the male. Syromastes is indeed a case in which
the spermatogonial number is an even one (twenty-two), while there is
a heterotropic chromosome in the second division. Half the sperma-
tozoa seem to receive ten chromosomes and half eleven, as in so
many other species of Coreidae. But as Gross also correctly de-
scribed, the heterotropic chromosome is here a bivalent which
represents two chromosomes united together. The true numbers
characteristic of the two classes of spermatozoa are therefore
ten and twelve, respectively. For the sake of clearness I will here
point out that this becomes at once intelligible under the assump-
tion that the female number is not twenty-two, as Gross believed,
but twenty-four; and such I believe will be found to be the fact.
That Gross was mistaken doubtless misled by the earlier
conclusion of Paulmier ('99), in which he was at first followed by
Montgomery ('01) in supposing that the chromosome nucleolus
of the growth period divides to form the m-chromosomes, is I
think thoroughly demonstrated by my preparations. In the case
of Anasa and Alydus I showed ('o5c) that the m-chromosomes are
not formed in the way Paulmier believed, but arise from two small
separate rod-like chromosomes that are in a diffused condition
during the growth period and only condense to form compact
bodies at the same time that the condensation of the larger chro-
mosomes takes place. I have since found this to be true of many
other species. It is confirmed in the case of Anasa by the smear
preparations of Foot and Strobell ('07), and I have also since fully
established the same conclusion by this method, by means of
which every chromosome in the nucleus may be demonstrated. 5
5 This is opposed to the conclusion of Montgomery ('06).
74 Edmund B. Wilson
Although I have no smear preparations of Syromastes it is perfectly
clear from the sections that the facts are the same here as in Anasa
Alydus, and other forms. In the early prophases of the first divi-
sion (at a period corresponding to Gross's Figs. 31 to 37) when the
plasmosome has disappeared or is greatly reduced in size, the
nuclei contain both the chromosome-nucleolus and the m-chro-
mosomes. This is shown in great numbers of cells with unmis-
takable clearness and after various methods of staining, particu-
larly after safranin alone or combined with lichtgriin. In the
early part of this period the chromosome nucleolus is at once
recognizable by its intense color and sharp contour and is not
for a moment to be confused with a plasmosome. The ordinary
bivalents are still in the form of ragged pale bodies, having the
form of longitudinally split rods or double crosses. The m-chro-
mosomes have the same texture and staining reaction, but are
much smaller and never show the cross form. While it is diffi-
cult to show the facts to demonstration in photographs of sections
they may be fairly well seen in the following. Photo 18 shows the
chromosome nucleolus (not quite in focus,) one of the large biva-
lents (two others barely appear) and both m-chromosomes.
Photo 19 is a similar view (the m-chromosomes more condensed),
while Photo 20 shows the m-chromosomes and three of the ordi-
nary bivalents. The succeeding changes must be rapidly passed
through, since the successive steps are often seen in the same cyst,
passing from one side to the other. In these stages the large
bivalents rapidly condense and regain their staining capacity,
finally assuming a bipartite or quadripartite form. The m-chro-
mosomes undergo a similar condensation, being finally reduced to
ovoidal or spheroidal bodies. The chromosome nucleolus, on
the other hand, becomes somewhat looser in texture and assumes
an asymmetrical quadripartite shape, in which form it enters the
equatorial plate to form the eccentric "accessory" chromosome.
The period at which the m-chromosomes condense varies consider-
ably, and the same is true of their relative position; sometimes they
are in contact, sometimes more or less widely separated, even lying
on opposite sides of the nucleus. Photo 21 shows two nuclei,
one above the other, in each of which appear both m-chromosomes,
Studies on Chromosomes 75
the chromosome nucleolus and a number of the other bivalents.
Photo 22 shows the same condition. Photo 23, from the same cyst,
is slightly later, showing the two spheroidal ra-chromosomes wide
apart, the chromosome nucleolus, and several of the other chro-
mosomes. (The chromosomes nucleolus, perfectly recognizable
in the preparation, is in the photograph hardly distinguishable
from the other bivalents seen endwise.) Up to this point, which
shortly precedes the dissolution of the nuclear membrane, the
chromosome nucleolus is still immediately recognizable by its
deeper color (especially after safranin). There follows a brief
period in which this distinction disappears, but the chromosome
nucleolus is still recognizable by its asymmetrical form. That it
gives rise to the eccentric "accessory" is, I think, beyond doubt.
The evidence is demonstrative that it does not divide to form the
m-chromosomes, and that the latter arise from separate rods as
described. Gross appears to have seen these rods at a much
earlier period (cf. his Fig. 10) and correctly identifies them with
the spermatogonial m-chromosomes; but he believed them to give
rise to the "accessory."
The relation of the chromosome nucleolus to the spermato-
gonial chromosomes cannot be determined in Syromastes with the
same degree of certainty as in Pyrrochoris (as described beyond) ,
but the size relations leave hardly a doubt that Gross was right
in asserting its origin from two of the larger of these chromosomes.
The study of these relations is of importance because I believe
they justify the conclusion that the chromosome nucleolus, and
hence the "accessory," is nothing other than a pair of slightly
unequal idiochromosomes, which can readily be recognized in the
spermatogonial groups.
Study of the spermatogonial groups in detail shows that twenty
of the chromosomes may be equally paired, while the remaining
two are slightly but distinctly unequal in size. These can always
be recognized as the smallest of the chromosomes except the
m-chromosome . Photos I and 2 show two groups in which this
clearly appears. These photographs are reproduced in Text Figs.
i<3, ib, with two others, c and d, the chromosomes in question
being designated as I and /'.
7 6
Edmund B. Wilson
It is evidently this pair that give rise to the bivalent "accessory"
(eccentric) chromosome of the first division and hence to the
chromosome nucleolus of the growth period. Gross correctly
describes this bivalent as a quadripartite body or tetrad, but
overlooked the fact that it is composed of two slightly unequal
halves, and these correspond in relative size to the unequal pair
in the spermatogonia. This appears unmistakably in a great
number of polar views of the first division metaphase (though it
is not always apparent) and is clearly shown in Photos 3, 4 and
7b Note on the Chromosome groups of Metapodius and Banasa.
Biol. Bull.
'070 The Supernumerary Chromosomes of Metapodius. Read before
the May Meeting of the N. Y. Acad. of Sci. Science,
xxvi, 677.
'08 The Accessory Chromosome of Anasa tristis. Read before the Am.
Soc. of Zoologists, December, '07. Science, xxvii, 690.
EXPLANATION OF PLATES
All of the figures are reproduced directly from photographs by the author, without retouching. The
originals were taken with a Spencer -fa oil-immersion, Zeiss ocular 6, which gives an enlargement of 1500
diameters. The admirable method of focusing devised by Foot and Strobell was employed. They
are reproduced at the same magnification.
PLATE I
(Photos i to 5, 10 to 23, Syromastes marginatus; 6 to 10, Metapodius terminalis; 24 and 25, Pyrro-
choris apterus).
i and 2. Spermatogonial groups of Syromastes; copied in Text-fig, i, a, b.
3 to 5. Polar views, first maturation metaphase; w-chromosome at the center, idiochromosome-
bivalent ("accessory" chromosome) outside the ring at the left.
6 and 7. Corresponding views of Metapodius, typical condition with the two separate idiochromo-
somes outside the ring at the left.
8 and 9. The same; exceptional condition, with the idiochromosomes (at the left) in contact.
10. Polar metaphase, second division, Syromastes.
II to 17. Side views of the same division. The duality of the idiochromosome appears in 12, 16
and 17.
18 to 23. Early prophases of first maturation division, Syromastes. Each of these shows the
separate wz-chromosomes, and in all but No. 20 the chromosome nucleolus (idiochromosome bivalent)
also appears.
24 and 25. Spermatogonial metaphases of Pyrrochoris (copied in Text-figs. 2, a, i).
UDIES ON CHROMOSOMES
Edmond K. VHlsor.
PLATE 1
'
t
-
lit
. c; ^
The Journal of Experimental Zoology, Vol. VI, No 1.
\VII.SON, I'HOTO
X'.E. A defect In the plate (not preaen
one chromosome at the right side of Eh<
shows it correctly.
PLATE II
Pyrrochoris apterus
26 to 31. Spennatogonial groups, each showing twenty-three chromosomes, including the large
unpaired idiochromosome; 30, 31 illustrate the rare case in which the latter appears double, owing to
marked sigmoid curvature. These photos are copied in Text-figs. 2, c, d, e, j, k and /, respectively.
32 and 33. Post-phases shortly following last spermatogonial division; the chromosomes still distinct,
idiochromosome recognizable by its large size and deeper color.
34 and 35. Presynaptic stages following the last, showing "caterpillar'' stage of idiochromosome
and small nucleoli. In the last two the shortening has begun.
36 to 38. Further condensation of the idiochromosome; initial stages of synizesis; apparent duality
of the idiochromosome in two of the cells.
39 to 42. Synizesis, showing various forms of the chromosome nucleolus.
43 and 44. Early post-synaptic stages.
45 and 46. Polar metaphases, first spermatocyte division.
47 to 49. Side views of second division.
50 and 51. Polar metaphases, second division.
i the original negative) causes
to appear OnVble. Fir.lj
PLATE II
hdmond H. \\nson.
2S 2v>
30 l
::?
*. i,r
The Journal of Experimental Zoology, Vol. VI, No I.
WILSON. I'HOTO.
$3 >
' EBM, 8, WILSON,
COLUMBIA UNIVERSITY, NtW YORK,
STUDIES ON CHROMOSOMES
V THE CHROMOSOMES OF METAPODIUS, A CONTRI-
BUTION TO THE HYPOTHESIS OF THE GENETIC
CONTINUITY OF CHROMOSOMES
By
EDMUND B. WILSON
REPRINTED FROM
THE JOURNAL OF EXPERIMENTAL ZOOLOGY
Volume VI No. 2
BALTIMORE, MD., U. S. A.
WILLIAMS & WILKINS COMPANY
STUDIES ON CHROMOSOMES
V THE CHROMOSOMES OF METAPODIUS. A CONTRI-
BUTION TO THE HYPOTHESIS OF THE GENETIC
CONTINUITY OF CHROMOSOMES 1
BY
EDMUND B. WILSON
WITH ONE PLATE AND THIRTEEN FIGURES IN THE TEXT
The genus Metapodius (Acanthocephala), one of the coreid
Hemiptera, shows a very exceptional and at first sight puzzling
relation of the chromosome-groups which has seemed to me worthy
of attentive study by reason of its significance for the hypothesis
of .the "individuality" or genetic continuity of the chromosomes.
The most conspicuous departure from the relations- to which we
have become accustomed lies in the fact that different individuals
of the same species often possess different numbers of chromo-
somes, though the number in each individual is constant. An
even more surprising fact is that in all of my own material every
male individual possesses at least 22 spermatogonial chromosomes,
including a pair of unequal idiochromosomes like those of the
Pentatomidae, while in Montgomery's material of M. terminalis
every male has but 21 spermatogonial chromosomes, one of which
is a typical odd or "accessory" chromosome (unpaired idiochro-
mosome). 2
The present paper presents the results of an investigation of
these relations that has now extended over nearly four years, in
the course of which serial sections of more than sixty individuals
1 Part of the cost of collecting and preparing the material for this research was defrayed from a grant
of $500 from the Carnegie Institution of Washington, made in 1906. I am indebted to Rev. A. H.
Manee, of Southern Pines, N. C., for valuable cooperation in the collection of material, and to Dr.
Uhler, Mr. Heidemann, Mr. Van Duzee, and Mr. Barber for aid in its identification.
2 By Professor Montgomery's courtesy I have been enabled to study thoroughly his original prep-
arations and to satisfy myself of the correctness of his account (Montgomery '06). I also owe to him
a number of unsectioned testes of the same type.
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. vi, NO. 2.
148 Edmund B. Wilson
have been carefully studied. These individuals belong to three
well marked species M. terminalis Dall. and M. femoratus Fab.
from the Eastern and Southern States, M. granules us Dall. from
the Western all of which show a similar numerical variation. 3
My first material, including sections of two testes of M. terminalis
(Nos. i, 2) from the Paulmier collection, long remained a complete
puzzle and led me to the suspicion that the material was patho-
logical. This possibility was eliminated by the study of additional
material of the same type; but the contradiction with Montgom-
ery's results on the same species suggested that his specimens
were not correctly identified (Wilson 'o/a). Continued study at
length convinced me that this supposition too was probably un-
founded. If the identification was correct, as I now believe it
was, M. terminalis is a species that varies not only in respect to
the individual chromosome number but also in respect to the sex-
chromosomes, certain individuals having an unpaired "accessory"
chromosome, while others have an unequal pair of idiochromo-
somes. The latter condition alone has thus far been found in
M. femoratus and M. granulosus. The essential facts, and the
general history of the spermatogenesis, are otherwise closely
similar in the three species.
The range of variation in the number of chromosomes is in
M. terminalis from 21 to 26, in M. femoratus from 22 to 27 or 28,
and in M. granulosus from 22 to 27, the particular number (or
its equivalent in the reduced groups) being a characteristic feature
of the individual in which it occurs. I do not mean to assert that
there is absolutely no fluctuation in the individual. In this genus,
as in others, apparent deviations from the typical number fre-
quently are seen, and real fluctuations now and then appear; but
the latter are so rare that they may practically be disregarded.
That the number may be regarded as an individual constant
(subject to such deviations as are hereafter explained (p. 185) is
abundantly demonstrated, not only by the agreement of large
numbers of cells from the same individual but perhaps even more
3 A complete list of the individuals examined, arranged by localities, is given in the Appendix at
p.-2O2, Each individual is there designated by a number by which it is referred to in the text and
description of figures.
Studies on Chromosomes
149
convincingly by the definite correlation of the spermatocyte-
groups with those of the spermatogonia of the same individual.
This is shown in the following table, which summarizes the facts
thus far observed. 4
SUMMARY
Somatic number
(spermatogonia or
ovarian cells)
First spermatocyte
division
terminalis
femoratus
granulosus
J
?
J
9
d 1
9
21
II
12
'3
14
15
16
i?
9
3
5
3
2
I
o
4
2
2
O
o
3
o
2
O
2
O
o
o
2
I
O
I
i
o
I
2
4
i
4
i
o
o
I
2
22
27
24.. . .
2C. . .
26
(27)
28 (Z7 ?)
Distribution in the whole group
Total somatic number
Number of males
Number of females
Totals
21
22
7
1 1
27. . .
7
1 1
14.
ii
2C. . .
j
j
6
26
7
10
(27)...
I
o
i
28 (27 ?) .
o
i
i
Total . .
AT
IQ
62
4 The somatic numbers of the males are in each case determined from the dividing spermatogonia.
Those of the female are from dividing cells in various parts of the ovary mainly from the region just
above or below the end-chamber some of them undoubtedly folicle-cells, others probably young nutri-
tive cells or obgonia. The chromosome-groups from different regions differ considerably in size, but
otherwis show the same general characters. With a very few exceptions the number of chromosomes
has been determined by the count of several groups from the same gonad, in many cases by the count of
a very large number. In many individuals hundreds of perfectly clear equatorial plates may be seen
and the evidence is entirely demonstrative. In seven of the males (owing to lack of mitoses, or to defec-
tive fixation) the somatic number has been inferred from that shown in the spermatocyte divisions, or
vice versa; but with a single exception both numbers have been directly observed in other individuals of
the same type. I am therefore confident that the numbers are substantially correct as given. In case of
the female, only the somatic numbers can be given, since the maturation-divisions are not available for
study.
150 Edmund B. Wilson
The material of terminalis is from New Jersey, Pennsylvania,
Ohio, North Carolina, South Carolina and Georgia; that of femor-
atus from the three states last named; that of granulosus from
Arizona. The variation of number is independent of locality, and
individuals of the same species showing different numbers were
often taken side by side on the same food plants. It is equally
independent of sex, as the table at once shows. I am unable to
find any constant correlation between the number of chromosomes
and any other visible structural characters of the adult animals.
Such an astonishing range of variation in the chromosome num-
ber in the same species seems at first sight to present a condition
of chaotic confusion. But, as I shall endeavor to show, the first
impression thus created disappears upon more critical examination.
Detailed study of the facts proves that the variation is not indis-
criminate but affects only a particular class of small chromo-
somes that are distinguishable from the ordinary ones both by
size and by certain very definite peculiarities of behavior. These
chromosomes are absent in all of Montgomery's material; in my
own they are sometimes present, sometimes absent, the total num-
ber varying accordingly. The chromosomes in question are the
ones which in earlier papers I have called the "supernumeraries." 5
In behavior they show an unmistakable similarity to the idiochro-
mosomes; and for reasons given beyond I believe them to be noth-
ing other than additional small idiochromosomes, the presence of
which has resulted from irregularities of distribution of the idio-
chromosomes in preceding generations. The relations seen in
Montgomery's material form the converse case, the small idio-
chromosome having disappeared or dropped out. I shall try to
show that both cases are probably due to the same initial cause.
5 Wilson 'oya, 'oyb. I first discovered this phenomenon in the pentatomid species Banasa calva
('05!)) describing the single supernumerary as a "heterotropic chromosome." Later ('oya) a single
supernumerary was found in certain individuals of Metapodius terminalis, and other numerical varia-
tions in this species and in femoratus and granulosus were briefly recorded; but at that time I did not
yet fully understand the facts. Banasa calva is the only form oustide the genus Metapodius, in a totalof
more than seventy species of Hemiptera I have examined, in which supernumerary chromosomes have
been found. Miss Stevens ('o8b) has recently found in the coleopteran genus Diabrotica a condition
that is in some respects analogous to that seen in Metapodius.
Studies on Chromosomes 151
A GENERAL DESCRIPTION
Since the phenomena as a whole are somewhat complicated, I
have thought it desirable to bring the most essential facts together
for ready comparison in a preliminary general account illustrated
by a limited number of selected figures (Figs. I, 2). The funda-
mental type of the genus is, I believe, represented by individuals
that possess 22 chromosomes in the somatic groups of both sexes,
and in which no supernumeraries are present (Fig. i, ) and a tripartite
chromosome-nucleolus formed by the idiochromosomes and the
supernumerary attached in a row (cf. Photo 27; additional figures
in Figs. 7-8). Fig. I, /-/ (terminalis, No. 43), show a 23-chro-
mosome group with one small supernumerary. This clearly
appears in the spermatogonial group (/); and the small idiochro-
mosome (/) is also distinguishable. In the nucleus from the
growth-period (/), the supernumerary and small idiochromosome
156 Edmund B. ffilson
are united (/, s) the large idiochromosome (/) being separate.
(Additional figures in Figs. 7, 8.)
Fig. 2, a-c (terminalis, No. 21), show the corresponding stages
in an individual of the 24-chromosome type, with two large super-
numeraries. Their identification in the spermatogonial group
is somewhat doubtful. (Additional figures in Fig. 10.)
Fig. 2, d, e (terminalis No. 34), show a 25-chromosome type
with three large supernumeraries. The growth-period (/) is from
an individual of granulosus (No. 54) that is possibly of the 26-
chromosome type. (Additional figures in Fig. 12.)
Fig. 2, g, h (femoratus No. 42), and z (granulosus, No. 60)
show the 26-chromosome type with four large supernumeraries.
(See Photo. 28, additional figures in Figs. 9, 10.)
Fig. 2, jl (femoratus, No. 40), are from a very interesting indi-
vidual of the 26-chromosome type, with two large and two small
supernumeraries (additional figures in Figs. 9, 10). The sperma-
togonia of this individual (k) uniformly show 26 chromosomes,
including four very small ones (two m-chromosomes, two small
supernumeraries), but the large supernumeraries and the small
idiochromosomes are doubtful. No case was found in which all
of the six components of the chromosome-nucleolus could be seen;
/ shows five of them, including the two small ones.
B ADDITIONAL DESCRIPTIVE DETAILS
I will now give a somewhat more detailed and critical account
of the facts. Taken as a whole, the series (including nearly 300
slides of serial sections) presents a profusion of evidence on many
cytological questions that could not be adequately described save
in a large monograph; but I will here limit the account mainly to
the numerical and topographical relations of the chromosomes.
The clearness of the preparations is such that nearly all the prin-
cipal phenomena might have been illustrated by photographs (of
which upwards of 200 have been prepared). Thirty of these
are reproduced in Plate I, less for the purpose of giving the
evidence in detail than of illustrating its character to those not
directly familiar with this material.
Studies on Chromosomes
s
a
Aft
d
* *
5
FIG. 2
et-e, M. terminalis; /, /, granulosus; g-h, j-l, femoratus.
a-c (No. 21), 24-chromosome form, two large supernumeraries.
d-e (No. 34), 25-chromosome form, three large supernumeraries.
/ (No. 54), growth-period, 25- or 26-chromosome form.
g-h (No. 42 Photo 8), 26-chromosome form, four large supernumeraries.
(No. 60), 26-chromosome form, growth-period.
j-l (No. 40), 26-chromosome form, two large and two small supernumeraries.
158 Edmund B. Wilson
I Individuals having twenty-one spermatogomal Chromosomes,
including an unpaired I dio chromosome. Small Idiochromo-
some and Supernumeraries absent
To this group belong only the specimens, all males, collected by
Montgomery at West Chester, Pa., of which I have examined nine
individuals, all of which have essentially the same characters. 7
Montgomery ('01) originally described these forms as having 22
spermatogonial chromosomes but subsequently ('06) corrected this
to 21, describing the phenomena as agreeing in all essential respects
with those seen in Anasa and other coreids. A study of the orig-
inal preparations has enabled me to confirm this later account in
every essential point. After the synizesis or contraction phase of
synapsis (as in all individuals of the genus) the ordinary chromo-
somes appear in the form of rather delicate spireme-like threads,
longitudinally split. In later stages of the growth-period they
shorten, become irregular, lose their staining capacity, and assume
the vague, pale condition characteristic of so many other forms.
In the early prophases of the first division they become more defi-
nite, stain more deeply, and appear as coarse longitudinally split
rods that often show an indication of a transverse division at the
middle point, or in the form of the double crosses as described by
Paulmier in Anasa ('99). In the later prophases they condense
still further to form nine compact bivalents which finally arrange
themselves in a more or less regular ring. The equatorial plate
of the first division always shows in polar view n chromo-
somes (Fig. 3,
/ ?
FIG. 4
22-chromosome forms
/j-/, first division; a, t, term. No. 19, typical (Photo 2); c, term. No. 12 (Photo 3); , Photo 25), or not infrequently widely separated (Fig. 6, c, d.
Photo 26). When in contact they form a double body closely
similar to the idiochromosome-bivalent of the second division.
There can be no question of confusing either of these bodies with
the plasmosome, since the latter, showing its characteristic stain-
ing reactions, is also present.
In the late prophases of the first division the idiochromosomes,
if previously united, almost invariably part company to divide as
separate univalents, as in other Hemiptera; but they usually
remain near together outside the principal ring. Only very excep-
tionally do they divide together.
The spermatogonial groups (Fig. 4, m-q) uniformly show 22
chromosomes, and in some cases the small idiochromosome may
be recognized by its small size (m, q). This is, however, not
nearly so marked as in the first division, since it now appears rela-
tively twice as large, owing to the univalent character of the other
chromosomes, and often it cannot certainly be distinguished from
the smaller of these (n, p).
These facts make it clear that if the small idiochromosome be
supposed to disappear, the entire series of phenomena would be-
come identical with those shown in the 2i-chromosome individuals,
the large idiochromosome now appearing as the odd or "acces-
sory" chromosome.
The unreduced female groups of this type (ovarian cells) are
closely similar to those of the male (Fig. 4, rt) but a small idio-
chromosome can never be distinguished. The absence of this
chromosome cannot be so convincingly shown in Metapodius as
in such forms as Lygaeus or Euschistus, owing to its greater rela-
tive size. Nevertheless, after the detailed study of many female
groups I am convinced that this chromosome is not present, and
that all the chromosomes may be equally paired. Apart from
analogy, therefore, I think the conclusion reasonably safe that
in Metapodius, as in other forms, the unequal idiochromosome-
pair of the male is represented in the female by a large equal pair,
Studies on Chromosomes
::.*:.:* A
' - '* ' <
urn.
w.
FIG. 5
22-chromosome forms
a-c, second division, polar view; a, fern. No. 19; b, fern. No. 28; c, gran., 47 (Photo 12).
d-p, second division, side view; d-h, fern. No. 29, metaphases, separation of idiochromosomes; i j,
term. No. 19, anaphases, lagging of one idiochromosome; k-m, gran., No. 47, late anaphases (Photo 17);
w, term., No. 19, late anaphase, lagging large idiochromosome; o, fern., No. 46, exceptional condi-
tion, both idiochromosomes passing to one pole (Photo 18); p, term. No. 19, similar form; q, r, term.,
No. 19, sister anaphase groups, from the same spindle; 3, t, fern., No. 29, the same.
1 66
Edmund B. Wilson
and that, accordingly, the usual rule holds in regard to fertiliza-
tion.
Exceptional conditions. There are two conditions, rarely seen,
that are of interest for comparisons with other species. Now and
then the idiochromosomes fail to separate for the first division,
but remain in more or less close union to form an asymmetrical
bivalent, which in side view is seen to form a tetrad (Figs. 4, /-/,
k y Photo 3). This bivalent undergoes an equation division, in
this respect agreeing with the conditions uniformly seen in Syro-
FIG. 6
M. femoratus (No. 29) 22-chromosome form
Four nuclei from growth-period showing diffused ordinary chromosomes, condensed chromosome-
nucleoli and plasmosome; in a and b the two idiochromosomes are united to form double chromosome-
nucleoli (Photo 25); in c and d they are separate (Photo 26).
mastes (Gross '04, Wilson '09), and differing from that occurring in
the Coleoptera or Diptera (Stevens '06, 'o8a). A rarer but more
interesting deviation from the type is the failure of the idiochro-
mosomes to separate in the second division, both passing together
to the same pole (Fig. 5, o, />, Photo 18). Since the other chromo-
somes divide equally it may be inferred that in this case one pole
receives 12 chromosomes and the other but 10. This has been
seen in only three cells and is doubtless an abnormality. It may
however, possess a high significance as forming a possible point
Studies on Chromosomes 167
of departure for the origin of the whole series of relations observed
in the genus.
3 Individuals possessing twenty-three Chromosomes; one
Supernumerary
This condition exists in all three species and has been found in
seven males and four females. In four of these males the super-
numerary is large (of approximately the same size as the small
idiochromosome, as in Fig. i, -/'); in three it is no larger than the
ra-chromosomes (as in Fig. I, /-/), and is indistinguishable from
the latter save in behavior. In each case, as already described,
the spermatogonia show 23 chromsomes and the first division 13;
and in those showing a small supernumerary in the first division
the spermatogonia always show three very small chromosomes.
The grouping in the first division, though conforming to the
same general type, shows many variations of detail, as may be seen
from Fig 7, a-l, Photos 4-6. It is a curious fact that the form of
grouping is to some extent characteristic of the individual. For
example, the typical arrangement, with both idiochromosomes
and supernumerary outside the ring, is very common in Nos. 43
(Fig. i, /-/) and 20 (7, a-c), very rare in Nos. I, 2 (Fig. 7, /)and
49 (Fig. 7, f-h). In No. 49, very many of the first division meta-
phases show both supernumerary and small idiochromosome
lying inside the ring (Fig. 7, g-h). I am unable to suggest an
explanation of this.
In this division all the chromosomes divide equally (Fig. 7, m-p),
so that each secondary spermatocyte receives 13 chromosomes.
The usual regrouping now takes place, and the idiochromosomes
couple as usual to form, an asymmetrical bivalent. The super-
numerary sometimes remains free (i. e., not attached to any other),
in which case 12 chromosomes appear in polar view (Fig. 8, b,d).
Much more frequently the supernumerary attaches itself to the
idiochromosome bivalent to form a triad element, polar views now
showing but II chromosomes (8, a, c\ one of which is compound.
The three components of such triads usually lie in a straight line,
the supernumerary being attached sometimes to the small idio-
1 68 Edmund B. Wilson
FIG. 7
23-chromosome forms, one supernumerary
a-h, first division, polar views, one large supernumerary; a-c, term., No. 20, typical grouping; de<
gran., No. 48; /, g, h, gran., No. 49 (Photo 5).
'-/, first division, polar views, one small supernumerary; ;', term. No. i (Photo 6); j-l, term. No. 43
typical grouping in k.
m-p, first division, side-views; m and n (term. No. 43) show division of 7, /', m, and small s; o, term.,
No. 20, division of I, i, and large s; p, term., No. 43, division of m, i, and small s.
cf-s, spermatogonial groups from individuals with one large supernumerary; q, r, term., No. 20; s,
gran., No. 49.
t-y, spermatogonial groups from individuals with one small supernumerary; /, u, term., No. 43;
v-y, term., No. 2 (Photo 29).
Studies on Chromosomes
169
%'*^ **
**.+ -{ *
t* 7 % .. * m . %
r
. %
v%W
*
r * ^ w^ ^
>>5"' '?
r ^^
i/
t
'/-
^
*
^V/
^
^
ill'
>
//
FIG. 7
170
Edmund B. Wilson
chromosome, sometimes to the large, or not infrequently lying
between the two (Fig. 8, g, h, o-q}.
* \
* m \ -. ^7. m
i. * . ' tfV
t.% ) ;% V:
-^ *"^ "'V
FIG. 8
2 3- chromosome forms, one supernumerary
a-f, polar views, second division; a, gran., No. 49, large supernumerary attached; fe(same cyst)super-
numerary free; c-d, similar views of terminalis, No. 43, with small supernumerary; e-f (No. 43), sister
groups from same spindle, pclar views.
g-m, side-views, second division, from gran., No. 49, with large supernumerary, free in j, attached
in the others.
n-u, similar views from individual (term., No. 43) with small supernumerary; in u the supernumer-
ary is free.
Studies on Chromosomes 171
In the ensuing division, if the supernumerary lies free it passes
without division as a heterotropic chromosome to one pole (8, ).
When connected with the idiochromosome bivalent it passes to one
pole attached to one or the other of the idiochromosomes (Fig.
8, k-m, p-t). In either case one pole receives 1 1 chromosomes and
one 12 (Fig. 8, e, /); but since the supernumerary may accompany
either idiochromosome four classes of spermatid nuclei are formed,
namely:
(l) 10 =7=11 (2) 10 + / + S = 12
(3) 10 + t --= II (4) 10 + I + S = 12
As described in an earlier paper ('oya), there is a tendency for
the supernumerary to be associated more often with the small
idiochromosome than with the large, and classes I and 2 are accord-
ingly more numerous than 3 and 4. I was formerly inclined to
attribute importance to this as pointing to the more frequent
occurrence of the supernumerary in the male than in the female.
The larger series of data now available leads me to doubt whether
it has much significance; for if (leaving the 2i-chromosome forms
out of account) the whole series of forms be taken together, one
or more supernumeraries are found in 27 out of 34 males, and in
15 out of 19 females about 80 per cent in each case. It appears
therefore that in the long run the supernumeraries are distributed
between the two sexes with approximate equality.
Figs. 7, q-s show spermatogonial groups from individuals with
one large supernumerary, but in none of them can this chromosome
or the small idiochromosome be certainly distinguished. Fig.
7, t-y are from individuals with one small supernumerary, each
showing three very small chromosomes. In t and u the small
idiochromosome is doubtful. Fig. 7, v-y, on the other hand, are
from an individual (terminalis, No. 2), showing great numbers of
very fine spermatogonial groups, in almost all of which the small
idiochromosome is at once recognizable. The same is true of a
second individual from the same locality. These two individuals,
from the Paulmier collection, were the first material I examined
and found so puzzling until the examination of another similar
individual, No. 43, cleared up the nature of the second division.
172 Edmund B. Wilson
4 Individuals with twenty-six Chromosomes; four
Supernumeraries
It will be convenient to consider this type before the 24- and 25-
chromosome forms, since the material is more favorable for an
account of the remarkable phenomena occurring in the second
division. Of these individuals there are seven males and three
females, all three species being represented. Unfortunately very
few perfectly clear spermatogonial groups are shown; but the
spermatocyte-divisions and cells of the growth-period are particu-
larly well shown and in large numbers of cells. In all but one of
these individuals the four supernumeraries are large and of nearly
equal size. In one (femoratus No. 40) two are large and two
small. The latter case, already shown in Fig. 2, /-/, is further
illustrated by Fig. 9, A, /', /, n, o. Two of these (h and /) show
but three supernumeraries in the first division, a common appear-
ance in this individual (see p. 186). Fig. 9, a-/, show varying
arrangements of the 16 chromosomes that appear in the first
division, the most typical ones being k and /. In 9, a-c, k, /, both
idiochromosomes and the four supernumeraries lie outside the
ring. In 9, g, all but the large idiochromosome are inside the
ring.
In some of these slides the compound chromosome-nucleoli are
shown with great distinctness in many cells of the growth-period.
This body usually has the form of a flat plate that lies next the
nuclear wall (Fig. 10, q, r) so that a clear view of all the compo-
nents can only be had in tangential sections. Thus viewed (Fig.
10, s-u, Photo 28) it may often be seen to consist of six components
one of which (the large idiochromosome) is about twice the size of
the others and is usually at one side or end of the group. The
other five evidently represent the small idiochromosome and
the four supernumeraries. In side view (Fig. 10, q, r) not more
than three or four of the components, can as a rule be recognized.
In a considerable number of cases these six chromosomes are not
aggregated to form a single body but form two or more simpler
bodies.
The second division in these forms presents an extraordinary
Studies on Chromosomes
173
** ,
**& ft
%
h
-
r{
m
' '
J
'/
FIG. 9
26-chromosome forms, four supernumeraries
a-g, first polar, supernumeraries large and equal; a-d, fern., No. 42; e, gran., No. 55; /, gran., No.
59! & gran., No. 60.
h-j first polar, from (fern., No. 40, with two large supernumeraries and two small; all of these are
shown in;', (cf. Fig. 2,;), while in h and / one is missing (see p. 186).
k, first polar, term., No. 36; / from same individual (Photo 9).
m-o, spermatogonia groups; m, fern., No. 42, abnormal group with 27 chromosomes; n, o, fern.,
No. 40. showing two small supernumeraries.
p-q, ovarian groups, gran. No. 61.
174 Edmund B. Wilson
appearance which I at first thought must be due to an artificial
clumping together of the chromosomes through defective fixation;
but the study of very many of these figures convinced me that such
is not the case. As in the preceding types, ten of the chromosomes,
including the m-chromosomes, have the form of symmetrical
dumb-bell shaped bodies which are equally halved in the ensuing
division. The remaining chromosomes are usually aggregated to
form a compound element (Fig. 10, /z-/, Photos 22, 23) in which
may be very clearly distinguished the same components as those
that appear in the chromosome-nucleoli of the growth-period;
and the size-relations make it evident that one of them is the large
idiochromosome, one the small, while four are the supernumer-
aries. In other words, these six chromosomes, which divide as
separate univalents in the first division, have now again conju-
gated to form a hexad group. This compound element almost
always lies near the center of the group. Polar views of this divi-
sion accordingly show typically n chromosomes, of which the
central one is compound (Figs. 10, a-g, Photo 13). Not infre-
quently, however, one or more of the supernumeraries may be sep-
arate from the others (Fig. 10, f, g), the apparent number in polar
view varying accordingly.
In side views the grouping of the components of the hexad
element is seen to vary considerably though the large idiochro-
mosome is more frequently at one end of the group. In the ensu-
ing division the other ten chromosomes divide equally, while the
hexad element breaks apart into two groups that pass to opposite
poles (Fig. 10, /-/>). The distribution of the various elements
is difficult to determine exactly, since they always lag behind the
others in the anaphases and are scattered along the spindle in such
a way as often to give confusing pictures. The study of many such
anaphases leads me to conclude, however, that at least one of the
smaller components always passes to the opposite pole from the
larger one, while the other four undergo a variable distribution.
In Fig. 10, /, the group is just separating into three toward each
pole; in 10 m, it is quite clear that three of the small ones are pass-
ing to one pole, while the large one and two small ones are passing
to the other, and Fig. 10, n, is probably a similar case. In these
Studies on Chromosomes 175
cases it seems clear that each pole receives 13 chromosomes, as
follows :
a 10 + 7 + 2* = 13 b 10 + i + 2s = 13
Fig. 10, o, on the other hand, shows a perfectly clear case in
which the hexad element has separated into a 2-group and 4-group:
Fig. 10, /?, shows what is probably a later stage of the same type.
In both these cases one pole appears to receive 12 and one 14 as
follows :
a 10 + / + 3* + 14 b 10 + ; + s = iz
one pole receiving but one supernumerary, and the other three.
The cases in which all of the components may be clearly recog-
nized in the anaphases are comparatively rare, and in the greater
number of them the distribution of the supernumeraries appears to
be symmetrical. Of their unsymmetrical distribution in some
cases there can be no doubt (and the same is true of the T4-chromo,
some form, as described beyond). The few undoubted cases of
this all show one to one pole and three to the other (as in Fig. 10, o-
p), and I have never found a case in which all four pass to the same
pole.
It seems, therefore, probable that in the 26-chromosome type
there are at least six classes of spermatozoa, as follows:
(l) 10 + I + 2s = 13 (l) 10 + ; + 2s 13
(3) 10 + 7 + S = 12 (4) 10 + I + 35 = 14
(5) 10 + 7 + 3* = 14 (6) 10 + / + j = 12
It is possible that the following four additional classes may be
produced :
(7) 10 + 7 + 4* = 15 (8) 10 + ; =n
(9) 10 + / =11 (10) 10 + + 45 = 15
Perfectly clear spermatogonial figures of this type were rarely
found, though many of them show approximately 26. The nor-
mal group of fern., No. 42, is shown in Fig. 2, h. Two groups from
fern. No. 40 (with two small and two large supernumeraries) are
shown in Fig. 9, n, o, each having 26 chromosomes including four
small ones (cf. Fig. 2, k). Two ovarian groups from gran., No. 61,
176 Edmund B. Wilson
FIG. 10
26-chromosome forms
tt-gj second division, polar, d from fern. No. 40, the others from fern. No. 42; a, (Photo 13) b, c,
show a single central hexad; in e and g the components are more loosely united; in d and /one supernumer-
ary is free.
h-p, side-views, second division, from fern. No. 42 (Photos 22, 23) explanation in text.
q-u, growth-period, gran.. No. 60; q and r show the compound chromosome-nucleolus in oblique
and side-view, s, t, u, en face.
Studies on Chromosomes
'77
178 Edmund B. Wilson
FIG. n
24-chromosome forms, two supernumeraries.
a-e, term., No. 21, first polar, showing various groupings; g, the same, gran., No. 52 (Photo 7).
h, term., No. 21, second polar, tetrad element near center.
i-o, somatic groups from individuals with two large supernumeraries; /-/, spermatogonial groups
from term. No. 21; m, n, ovarian groups from fern. No. 3150, ovarian group, fern., No. 45.
p-r, spermatogonial groups from fern., No. 22, with one large supernumerary and one small; Photo
30).
s-w, second division, side-view; s t term., No. 21 ; t-w, gran., No. 52 (Photo 21).
Studies on Chromosomes
179
i TV. .%*** %
* ' * * ' ** % *
J
W
r
c
*
g
p
illl
U "'*
m
m
W
W:
i
V ^//W
FIG. ii
180 Edmund B. Wilson
are shown in Fig. 9, /?, q. Fig. 9, m, shows a spermatogonial group
from fern., No. 42, that is abnormal in showing with perfect clear-
ness 27 instead of 26 chromosomes (cf. Fig. 2, /z).
5 Individuals with twenty-four Chromosomes; two
Supernumeraries
The material for these individuals and those of the 25-chromo-
some class, is less satisfactory than in the preceding case, but the
relations are undoubtedly quite analogous to those just described.
The 24-chromosome class is represented by 9 males and 4 females,
and occurs in all three species. In one of the males one of the
supernumeraries is large (of the same size as a small idiochromo-
some) and one small; in all the others both are large. Additional
figures of the first division, showing variations in the grouping, are
given in Fig. II, a-g; of spermatogonial groups in Figs, n, i-r.
Of particular interest is the male, term., No. 22, shown in Photo 30
and in Fig. 1 1, p-r. This individual was, unfortunately, immature
showing only spermatogonia and cells in the growth-period; but
many perfectly clear spermatogonial groups are shown. These
groups uniformly show 24 chromosomes, of which three are very
small, while in many cases two others are slightly but distinctly
smaller than the others. The latter are evidently the small idio-
chromosome and the larger supernumerary, while the three small
ones represent the w-chromosomes and the small supernumerary.
In the second division the two idiochromosomes and the super-
numeraries are frequently united to form a tetrad element, various
forms of which are shown in Fig. n, s-w. The distribution of
these four components is not so well shown in this material as in
that of the 26-chromosome class, described above. It is, however,
clear that this distribution is inconstant. In cases like those shown
in Fig. n, j, t, it is probable that the tetrad divides in the middle,
so that each idiochromosome is accompanied by a supernumerary,
and each pole receives 12 chromosomes. The cases shown in
Fig. n, ,
/ J W k
FIG. 13
27 and ( ?) zS-chromosome forms
a-i, first division, from gran., No. 57, having four large supernumeraries and one small j (polar)
and k (side-view), second division, same individual (Photo 10).
/, ovarian group from fern., No. 33, having three large and two or three small supernumeraries; in
this group appear 28 chromosomes.
8 Individuals with twenty-eight (?) Chromosomes; six
Supernumeraries.
The last case to be considered is that of a single female of femora-
tus (No. 33), in which the number is either 27 or 28. A single
perfectly clear ovarian group, shown in Fig. 13, /, shows beyond
184 Edmund B. Wilson
doubt 28 chromosomes, including five smallest ones and three or
four next smallest. A few other less clear groups were seen in
which appear but 27 chromosomes, the missing one being one of
the smallest. In these cases one of the small ones may be hidden
among the larger ones; but it is also possible that the 28-group is an
abnormality. In this individual there are probably three larger
supernumeraries and either two or three small ones.
C SUMMARY AND CRITIQUE
1 In the genus Metapodius the number of chromosomes is
constant in the individual but varies in different individuals from
21 to 27 or 28. The number 21 appears only in the males of M.
terminalis (Montgomery's material).
2 The number is independent of sex and locality, and is not
correlated with constant differences of size or visible structure in
the adults.
3 The variation affects only a particular class of chromosomes.
4 The 22-chromosome forms represent the type from which all
the others may readily be derived. These forms possess a pair of
unequal idiochromosomes which show the same behavior as in
Lygaeus or Euschistus, all the spermatozoa receiving n chromo-
somes, and half containingthe large idiochromosome, half the small.
5 In the 2i-chromosome forms the small idiochromosome has
disappeared, leaving the large one as an "odd" or "accessory"
chromosome. Half the spermatozoa accordingly receive 1 1 chro-
mosomes and half 10.
6 Numbers above 22 are due to the presence of from one to five
or six additional small chromosomes which show in every respect
the same behavior as the idiochromosomes, and are probably to be
regarded as additional small idiochromosomes. In the growth
period they have a condensed form and are typically associated
with the idiochromosomes to form a compound chromosome-
nucleolus. In the first division they divide as separate univalents.
In the second, they are typically (though not invariably) again
associated with the idiochromosomes to form a compound element.
The components of this element undergo a variable distribution
Studies on Chromosomes 185
to the spermatid nuclei. All the spermatid nuclei receive the
haploid type-group of II chromosomes, half including the small
idiochromosomes and half the large; but in addition each may
receive one or more supernumeraries. The total number of
chromosomes in the sperm nuclei is therefore variable in the same
individual.
7 Both the number of the supernumeraries and their size, indi-
vidually considered, are constant in the individual.
The first question that the foregoing report of results will raise
is whether the number and size relations of the chromosomes in
each individual are really as constant as I have described them.
I have for the most part selected for illustration and description
the more typical conditions ; but, granting the accuracy of the figures,
does such a selection really give a fair presentation of the actual
conditions ? It is almost needless to say that very many cases
might have been shown that would seem to give conflicting results.
By far the greater number of these discrepancies are, I believe,
only apparent. Numerical discrepancies of this kind are very
often evidently due to mere accidents of sectioning or to the super-
position or close contact of two or more chromosomes. Again,
apparent discrepancies in the size relations of the chromosomes, as
seen in polar views, very often arise through different degrees of
elongation (particularly in the^ maturation divisions). But apart
from such apparent variations, real deviations undoubtedly occur
in almost all of the relations described. Now and then, for exam-
ple, a spermatogonial or ovarian group is found that clearly shows
one chromosome too many (as in Fig. 9, m), 9 and the same is true
of the first spermatocyte-division, but such cases are very rare.
The former case is probably a result of an abnormality in the forma-
tion of the chromosomes from the resting nucleus, the latter not
improbably to a failure of synapsis. Again, both spermatogonial
and spermatocyte-cysts are occasionally found in which the num-
ber of chromosomes is doubled or quite irregular. These are
9 A perfectly clear case of this has been found in the pyrrochorid species Largus cinctus (a particu-
larly fine form for study). In this form the normal male number is n, the female 12; but in one
female three cells were found each of which shows with all possible clearness 13 chromosomes, very
many other cells showing the normal number.
1 86 Edmund B. Wilson
probably due to an antecedent nuclear division without cell divi-
sion, or to multipolar mitoses such as now and then occur in both
spermatogonia and spermatocytes.
As regards the chromosome-nucleoli of the growth-period, the
contrast between those of the 21 and 22-chromosome forms, or
between either of these forms and those with higher numbers is
usually at once apparent; but in very many cases where more than
one supernumerary is present the number of components can only
here and there be clearly seen. Contrary to what might be expected
from their compact form, the compound chromosome nucleoli
seem to be rather difficult of proper fixation, their components
often clumping together or breaking up more or less when they
coagulate. I infer this from the fact that different slides differ
materially in the clearness with which these bodies are shown.
Two discrepancies, apparent or real, should be especially men-
tioned. One is the difficulty of recognizing the larger supernu-
meraries in the somatic groups. As already explained, these chro-
mosomes, like the idiochromosomes, appear relatively much
larger in the somatic groups than in the first maturation division
(owing to their univalence in the latter case) ; but we should expect
to recognize them more clearly, at least in the female groups, than
is actually the case. This is perhaps due to their undergoing a
greater degree of condensation than the others during the growth-
period; but I am not sure that this explanation will suffice. A
second discrepancy, which may involve an important conclusion,
is that in perfectly clear views of the first division, the number of
supernumeraries is often less than would be expected from the
spermatogonial groups. This is notably the case with femoratus,
No. 40 (Fig. 9, h-f), which has clearly 26 spermatogonial chro-
mosomes, but very rarely shows 16 in the first division, the usual
number being 15. A similar discrepancy has been noted in other
individuals, and in several of the types. Since the typical num-
ber in all these cases appears in some or many of the first sperma-
tocytes, I long supposed the occasional deficiency to result from an
accident of sectioning. I now incline to believe, however, that in
some cases one (or possibly more) of the supernumeraries may
really disappear (by degeneration ?) during the growth-period,
Studies on Chromosomes 187
and that this may be one way in which their progressive accumu-
lation in number in successive generations is held in check.
For the foregoing reasons it cannot be said that any of the rela-
tions described appear with absolute uniformity or fixity. The
condition typical of each individual must be discovered by the
study and comparison of large numbers of cells. I will only say
that prolonged and repeated study has thoroughly convinced me
that the relations, as described, may be regarded as being on the
whole individual constants. This judgment is based primarily
on the exhaustive study of a few of the best series of preparations
of individuals of the 21, 22, 23, and 26-chromosome types, in
which the facts are quite unmistakable and have given the point
of view from which the less favorable material of other cases may
fairly be examined.
D DISCUSSION OF RESULTS
The principal significance of these phenomena seems to me
to lie in their bearing on the general hypothesis of the "individual-
ity" or genetic continuity of the chromosomes; but they are also
of interest for a number of more special problems which I will
first briefly consider.
The Relation of the Chromosomes to Sex-production in Metapodius
The conditions seen in this genus seem to be irreconcilable with
any view that ascribes the sexual differentiation to a general quanti-
tative difference of chromatin, whether expressed in the number or
the relative size of the chromosomes. In all known cases of constant
sexual differences in the chromosomes it is invariably the female
that possesses the larger number of chromosomes or the greater
quantity of chromatin, 10 and this has naturally suggested the
view that this difference per se may be the sex-determining factor.
As I have pointed out before ('09), such a view is inapplicable to
cases like Nezara or Oncopeltus, where the idiochromosomes are
of equal size and no quantitative sexual differences are visible;
yet the phenomena in these genera are otherwise so closely similar
10 See review in Wilson '09.
1 88 Edmund B. Wilson
to those seen in other insects that I cannot doubt their essential
similarity also in respect to sex-production.
In Metapodius the facts are still more evidently opposed to the
quantitative interpretation. The number of chromosomes has
here no relation to sex-production; and, as will be seen from the
table at p. 149, in the forms with supernumeraries the relative
frequency of high numbers and of low is nearly equal in the two
sexes. If my general interpretation of the chromosomes in this
genus be correct, a like conclusion applies to the total relative
mass of chromatin in the two sexes; for all individuals alike possess
the type-group of 22 chromosomes (Montgomery's form excepted)
while the supernumeraries represent the excess above this amount.
I have endeavored to determine whether this appears in direct
measurements, independently of my general interpretation; but
have found this impracticable for several reasons. Very consider-
able differences in the apparent size of the chromosomes are pro-
duced by different degrees of extraction; but this will not account
for the considerable differences seen in the same slide when the
extraction is uniform. It is evident that the actual size of the
chromosomes varies with the size of the cells; for example, both
in Metapodius and in many other genera, the chromosomes in
the larger spermatogonia near the tip of the testis are larger
(in many cases much larger) than those of the smaller spermato-
gonia of other regions. How great the differences are may be
appreciated by a comparison of the figures. For example, in
the spermatogonial groups of No. 2 (23 chromosomes, Fig. 7,
u-x), the chromatin mass is obviously much greater than in
those of No. 21 (24 chromosomes, Fig. II, /-/). In the 25-chromo-
some female groups shown in Fig. 12, i-k (No. 27), the chromatin
mass is evidently much less than in the 2i-chromosome male group
shown in Fig. I, &, or in the 23-chromosome male groups of Fig. 7,
v-x. Conversely, the 22-chromosome female group of No. 44
(Fig. 4, j) shows a much greater chromatin mass than in the corre-
sponding male group of No. 46 (Fig. 4, o), or the male 24-chromo-
some group shown in Fig. n, j.
Evidently, therefore, the relative mass of chromatin can only
be determined by means of accurate measurements of both the
Studies on Chromosomes 189
chromosomes and the mass of protoplasm, but I have found the
errors of measurement of the cell size to be too great to give any
trustworthy result regarding the relative chromatin mass.
Despite the difficulties in the way of an accurate direct deter-
mination, I believe the facts on the whole warrant the conclusion
that the relative chromatin mass shows no constant correlation
with sex. The most probable conclusion is that the male-produc-
ing spermatozoa in Metapodius are distinguished by the same
characters as in other forms having unequal idiochromosomes,
the former class being those that receive the large idiochromosome,
the latter those that receive the small one, irrespective of the super-
numeraries that may be present in either class. For reasons that
I have elsewhere stated, I believe that if the idiochromosomes be
the sex-determinants their difference is probably a qualitative
one, and since the small idiochromosome may be lacking it would
seem that the large one must in every case play the active role
perhaps as the bearer of a specific substance (enzyme ?) that calls
forth a definite reaction on the part of the developing individual.
If this be so, we can comprehend the fact that the presence of
additional small idiochromosomes (supernumeraries) in either
sex does not affect the development of the sexual characters in that
sex.
b The possible Origin of the unpaired Idiochromosome ("odd"
or "accessory" Chromosome) and of the Supernumeraries
The explanation of the unpaired idiochromosome offered in the
second and third of my " Studies on Chromosomes" ('05, '06) was
suggested by the fact that various degrees of inequality exist in the
paired idiochromosomes, there being an almost continuous series
of forms connecting those in which the idiochromosomes are
equal (Nezara, Oncopeltus) with those in which they are so very
unequal that the small one appears almost vestigial (Lygaeus,
Tenebrio). It is evident that by the further reduction and final
disappearance of the small member of this pair the large one would
be left without a mate, and its history in the maturation process
would become identical with that of an "odd" or "accessory"
190 Edmund B. Wilson
chromosome. I still believe that this explanation may be applic-
able to many cases; but a different one seems more probable in the
case of Metapodius and perhaps may be more widely applicable.
This was suggested by the observation (p. 166) that in a very few
cases, in 22-chromosome individuals both idiochromosomes were
seen passing to the same pole in the second division. The rare-
ness of this occurrence shows that it is doubtless to be regarded in
one sense as abnormal. But even a single such event in an original
22-chromosome male, if the resulting spermatozoa were functional,
might give the starting point for the whole series of relations ob-
served in the genus, including the establishment of an unpaired idio-
chromosome. The result of such a division should be a pair of
spermatozoa containing respectively 10 and 12 chromosomes.
The former might give rise at once to a race having an unpaired
idiochromosome and the somatic number 21 in the male (as in
Montgomery's material). The latter might similarly produce an
individual having in the first generation a single supernumerary
chromosome and in succeeding generations an additional number.
This appears from the following considerations :
1 If a lo-chromosome spermatozoon, arising in the manner
indicated, should fertilize an egg of the 22-chromosome class (hav-
ing ii chromosomes after reduction) the result should be a male
containing 21 chromosomes, the odd one being the large idiochro-
mosome derived from the egg. Such an individual would be in
no respect distinguishable from those of Montgomery's material,
and would similarly form male-producing spermatozoa containing
10 chromosomes and female-producing ones containing II (includ-
ing the unpaired idiochromosome). A single such male, paired
with an ordinary 22-chromosome female, would suffice to establish
a stable race identical with the form found by Montgomery at
West Chester, Pa., the males having 21 chromosomes, the females
having 22, precisely as in Anasa or Leptoglossus. This seems to
me the most probable explanation of the conditions found in
Montgomery's material; and possibly it may explain the origin
of the unpaired idiochromosome in other cases as well.
2 The result of fertilizing the same type of egg by a spermato-
zoon from the 12-chromosome pole would be an individual having
Studies on Chromosomes 191
23 chromosomes (egg n + spermatozoon 12) including two
large idiochromosomes hence presumably a female and one
small. The eggs produced by such a female should after matura-
tion be of two classes, having respectively n and 12 chromosomes.
The 12-chromosome class would contain both a large and a small
idiochromosome, and if fertilized by ordinary n-chromosome
spermatozoa would produce individuals with 23 chromosomes,
male or female according to the class of spermatozoon concerned.
Such females would, as before, contain two large idiochromosomes
and one small. The males would contain one large and two small,
and would accordingly produce spermatozoa having either II or
12 chromosomes.
Now, such an additional small idiochromosome in the male
would be indistinguishable from a single "supernumerary chro-
mosome" as it appears in the 23-chromosome individuals in my
material. The resemblance is shown not only in size but also in
behavior; for, as I have shown, the supernumerary, like the
idiochromosome, forms a chromosome-nucleolus during the growth
period, it divides as a univalentin the first division, and in the sec-
ond is usually associated with the idiochromosome bivalent. A
single such supernumerary chromosome, once introduced into the
race would lead to the presence of additional ones in succeeding
generations. Thus, 12-chromosome eggs fertilized by 12-chromo-
some spermatozoa would give individuals (male or female) with
24 chromosomes, including two supernumeraries; and from these
might arise, through irregularities of distribution such as I have
described, gametes with n, 12, or 13 chromosomes, giving in the
next generation 22, 23, 24, 25 or 26 chromosomes according to
the particular combination established in fertilization." If this
11 Since the presence of an unpaired idiochromosome in some individuals and of supernumeraries
in others is assumed to be traceable to the same initial cause, we should naturally expect to find the two
conditions coexisting side by side, and in approximately equal numbers; but in point of fact the former
is very rare and was only found in one locality, while the latter is very common. This may constitute
a valid objection to my interpretation. It should be borne in mind, however, that abnormal divi-
sions of the kind assumed to form the starting point are very rare, and that an extremely minute propor-
tion of the total number of spermatozoa produced ever actually enter the eggs. The chances against
fertilization by either class of the original modified spermatozoa are therefore very great. Since only
sixty individuals have been examined it need not surprise us that one of the two conditions in question
192 Edmund B. Wilson
interpretation be correct, the origin of an unpaired chromosome
in certain individuals of this genus has been owing to the same
cause that has produced the supernumeraries. Since both condi-
tions coexist in the same species, along with that which may be
regarded as the original type (22 chromosomes) it may be conclu-
ded that Metapodius is now in a period of transition from the sec-
ond to the third of the types distinguished in my last study. It
seems quite possible that other species of coreids that now have
constantly an unpaired idiochromosome may have passed through
a similar condition, though in all of them thus far examined both
the small idiochromosome and the supernumeraries have dis-
appeared. In Metapodius, accordingly, the supernumeraries
may be regarded as on the road to disappearance. That such is
the case is rendered probable by the fact that their number does
not pass a certain limit, and is rarely more than four. The very
small chromosomes of this kind, so often observed, are perhaps
degenerating, or even vestigial in character. But aside from this,
attention has already been called to the probability that one or
more of the supernumeraries may be lost during the growth-period
(p.i86); and while this is not certain, it may well be that both
methods are operative in their disappearance.
The foregoing interpretation of the supernumeraries enables
us to understand why variations in their number are not accom-
panied by corresponding morphological differences in the soma-
tic characters; for they are but duplicates of a chromosome already
present and hence introduce no new qualitative factor. It can
hardly be doubted that some kind of quantitative difference must
exist between individuals that show different numbers, but none
has been more frequently met with. Another objection might be based on the different relations that
occur in Syromastes. In this form (see Wilson '09) the passage of both idiochromosomes to one pole
without separation is a normal and constant feature of the second division, yet no supernumeraries
appear in any of the individuals, and it is probable that the female groups contain two pairs of idio-
chromosomes like the single pair that appears in the male. We have no data for a conjecture as to how
such a condition can have arisen; but evidently the small idiochromosome does not in this case become
an erratic supernumerary but retains a definite adjustment to the other chromosomes. Still, I do not
consider this an obstacle to my interpretation of Metapodius, for it is'now evident that the history of the
idiochromosomes in general has differed widely in different species and families, even among the Hemip-
tera. We have thus far only made a beginning in their comparative study. [See Addendum, p. 200.]
Studies on Chromosomes 193
has thus far been discerned. Such a difference does not appear
in the size of the animals, for there are large individuals with no
supernumeraries and small individuals that possess them. An
interesting field for experiment seems here to be offered.
c The "Individuality" or Genetic Continuity of the Chromosomes
It is in respect to this much debated hypothesis that the facts
observed in Metapodius seem to me most significant and important.
It is evident' that the whole series of relations are readily intelligi-
ble if the fundamental assumption of this hypothesis be accepted.
Without such explanation they seem to me to present an insoluble
puzzle. The disposition to reject this hypothesis that appears in a
considerable number of recent papers on the subject will doubtless
lead to more critical and exhaustive observation of the facts; but
when it goes so far as to deny every principle of genetic continuity
in respect to the chromosomes, it is, I believe, a backward step.
This reaction perhaps reaches a climax in the elaborate and
apparently destructive criticism of Fick ('07) who considers the
hypothesis to be thoroughly discredited, and believes his analysis
to justify the conclusion: "Dass weder theoretisch noch sachliche
Beweise fur die Erhaltungslehre vorliegen, sondern dass im Gegen-
theil unwiderleghche Beweise gegen sie vorhanden smd, so dass es
im Interesse der Wissenschaft dringend zu wiinschen ist, dass
die Hypothese von alien Autoren verlassen wird" ('07, p. 112,
italics in original). I incline to think that this sweeping judgment
would have carried greater weight had Professor Fick, in certain
parts of his able and valuable discussion, taken somewhat greater
pains in his presentation of facts and shown a more judicial temper
in their analysis. 12 To some of the objections and difficulties
12 1 will give two specific examples of this. The experimental results of Moenkhaus ('04), on hybrid
fishes, which evidently form a strong support to the continuity hypothesis, are unintentionally but com-
pletely misrepresented in the statement at p. 75: "So berichtet Moenkhaus bei Fundulus-Monidia-
kreuzung (sic), dass sich die beiderlei (zuerst sehr verschiedenartigen) Chromosomen in der Regel
schon nach der zweiten Teilung nicht mehr unterscheiden lassen." But Moenkhaus's explicit statement,
based on the examination of "many thousand cells," is that even in the late cleavage "Nuclei showing
the two kinds of chromosomes mingled together upon the spindle are everywhere to be found" (op. cit.,
p. 48). Fick evidently had in mind the fact that the paternal and maternal chromosomes do not as a
rule retain their original grouping after the first two or three cleavages. His actual statement, however,
194 Edmund B. Wilson
brought forward in this critique reply has already been made by
Boveri ('07), Strasburger ('08), Schreiner ('08) Bonnevie ('08) and
others. Some of the difficulties are real, but an attentive study
of the matter will show that a large part of Pick's critique is
directed against the strict hypothesis of individuality and offers no
adequate interpretation of the essential phenomenon that requires
explanation. It may be admitted that many of the facts seem at
present difficult to reconcile with the view that the identity cf the
chromosomes as actual individuals is maintained in the "resting"
nucleus; and I have myself indicated (The Cell, 1900, p. 300) that
the name "individuality" was perhaps not the best that could have
been chosen. Certainly we have as yet comparatively little evi-
dence that the chromosomes retain their boundaries in the "rest-
ing" nucleus. It is evident that the chromosomes are greatly
diffused in the nuclear network, and it may be that the substances
of different chromosomes are more or less intermingled at this time.
Pick's "manoeuvre-hypothesis," which treats the chromosomes
of the dividing cell as temporary "tactic formations," may there-
fore be in some respects a more correct formulation of the facts
than that given by the hypothesis of "individuality" in the strict
sense of the term. But the last word on this question has by no
means yet been spoken. A new light is thrown on it by the recent
important work of Bonnevie ('08) which brings forward strong
evidence to show that in rapidly dividing cells (cleavage stages
of Ascaris, root-tips of Allium), although the identity of the orig-
(both here and in the later passage at p. 98) will wholly mislead a reader not familiar with Moenkhaus'
work, in regard to one of the most significant and important discoveries in this whole field of inquiry.
Hardly less misleading is Professor Pick's report of my own observations on the sex-chromosomes
of insects, which are stated as follows: "Wilson's Unterschungen beweisen eben sicker nur soviel, dass
bei einigen Insektengattungen constante Beziehungen zwischen dem Geschlecht und dem Vorhandensein
eines besonderen Chromosomenpaares bestehen, bei anderen Gattungen nichf (p. 90). I am confident
that those who are familiar with the researches referred to will not accept this as a fair statement of the
results. The fact is that in one form or other the sex-chromosomes are present in all of the forms that
I have examined (now upwards of seventy species) and that with various modifications all conform to the
same fundamental type. It is true that in two genera (Nezara and Oncopeltus) the sex-chromosomes
are equal in size, and hence afford no visible differential between the somatic groups of the two sexes;
but I especially emphasized the fact (cf. '06, pp. 17, 34) that these chromosomes are in every other
respect identical with those of other forms in which the size-difference clearly appears, and are connected
with the latter by a series of intermediate gradations that leaves no doubt of the essential uniformity of
the phenomena.
Studies on Chromosomes 195
inal chromosomes is lost in the "resting" nucleus after each mito-
'sis, each new chromosome nevertheless arises by a kind of endog-
enous formation within and from the substance of its predecessor.
In this way an individual genetic continuity of the chromosomes
can be directly followed through the "resting period" of the nu-
cleus. " Eine genetischeKontinuitat der Chromosomen nacheinan-
der folgender Mitosen konnte in der von mir untersuchten
Objekten teils sicher (Allium, Amphiuma) teils mit iiberwiegen-
der Wahrsheinlichkeit (Ascaris) verfolgt werden. Es ging aber
auch hervor, dass eine Identitdt der Chromosomen verschiedener
Mitosen mcht existtert, sondern dass jedes Chromosom in einem
fruher existierenden endogen entstanden ist, um wieder am Ende
seines Lebens fur die endogene Entstehung eines neuen Chromo-
soms die Grundlage zu bilden" (op. cit, p. 54). Whether this
particular conclusion will also apply to more slowly dividing cells
remains to be seen. But apart from this direct evidence it seems
to me that a denial of every form of genetic continuity between
the chromosomes of successive cell-generations which, despite
certain qualifications, seems to be the position of Fick and a num-
ber of other recent writers is only possible to those who are ready
to ignore some of the most obvious and important of the known
facts, especially those that recent research has brought to light
among the insects. The most significant of these are :
1 In Metapodius the specific number varies, while in the indi-
vidual both the number and the size-relations of the chromosomes
are constant.
2 In all species where the somatic chromosome-groups show
sexual differences in regard to the number and size-relations of
the chromosomes, exactly corresponding differences exist between
the male-producing and the female-producing spermatozoa.
Both these series of facts demonstrate that the "tactic forma-
tion" of a fixed number of chromosomes of particular size is not a
specific property of a single chromatin-substance as such, of the
species. It has been assumed by some writers that departures
from the normal specific number, such as appear in merogonic,
parthenogenetic, double-fertilized or giant (double) eggs, are the
result merely of departures from the normal quantity of chroma-
196 Edmund B. Wilson
tin." 13 If attentively considered the facts summarized above will,
I think, clearly show the inadequacy of such an explanation.
Why should a given quantity and quality of chromatin always
reappear in the same morphological form as that in which it
entered the nucleus ? Why, for example, in Metapodius should
the minute fraction of chromatin represented by a single small
supernumerary always reappear in the form of such a chromosome,
showing specific peculiarities of behavior, rather than as a corre-
sponding enlargement of one of the other chromosomes ? Why
should a larger excess always appear as a group of two, three, or
more supernumeraries that differ definitely in behavior from
the others and show constant size relations among themselves ?
Specifically, in individual No. 40, why should two small supernu-
meraries and two large ones always appear, rather than three large
ones ? In species where a constant quantitative chromatin-differ-
ence exists between the sexes, why should the excess in the female
always appear in the same form as that which appears in the female-
producing spermatozoa in one case as a large idiochromosome
instead of a small (Lygaeus), in another as an additional chromo-
some of a particular size (very large in Protenor, small in Alydus,
of intermediate size in Anasa), in a third case as three additional
chromosomes (Galgulus) ?
To these and many similar questions which the facts compel
us to consider, I am unable to find any answer on the merely
quantitative hypothesis. Each of them receives a simple and
intelligible reply under the view that it is the number, size, and
quality of the chromosomes that enter the nucleus that determine
the number, size, and mode of behavior of those that issue from
13 Fick's treatment of these cases is worth citing. "Es muss von vornherein als wahrscheinlich be-
zeichnet werden, dass unter den abnormen Umstanden, da einmal die Zahl der'Chromatin-Manoverein-
heiten' (im Sinne meiner Manoverhypothese gesprochen) in der Zelle erhoht ist, diese Zahl sich erhalt"
(p. 96). Why should the number be maintained ? Because, we are told, "Die Erhaltung der erhb'hten
Zahl und ihre regelmassige Wiederkehr bei den folgenden Teilungungen muss bei dem nun einmal
uber die Norm erhohten Chromosomenbestand der Zelle als der einfachere, kichter verstandliche Far-
gang erscheinen, als es ein besonderer, ein "Regulation" auf die Norm hervorbringender Akt ware."
To most readers this will seem like an argument for, rather than against, the hypothesis of genetic
continuity. But since it is obviously not thus intended I can discover no other meaning in the passage
than that with a given "bestimmte Chromatinmanoverart" characteristic of the species (p. 115) the num-
ber of chromosomes formed is proportional to the quantity of chromatin-substance.
Studies on Chromosomes 1 97
it. But such an answer implies the existence of a definite indi-
vidual genetic relation between the chromosomes of successive cell-
generations; and it is this relation, I take it, that forms the essence
of the hypothesis of genetic continuity, whether or not we include
in the hypothesis the assumption that the chromosomes persist as
"individuals" in the resting nucleus where their boundaries seem
to disappear. We might, for instance, assume that the chromo-
somes are magazines of different substances (e. g., enzymes or the
like) that differ more or less in different chromosomes, that are
more or less diffused through the nucleus in its vegetative phase,
but are again segregated out in the original manner when the
chromosomes reform. 14 We have, admittedly, but an imperfect
notion of how such a re-segregation may be effected, though the
conclusions of Bonnevie already referred to, constitute an impor-
tant addition to the earlier ones of Boveri (see '07, p. 232) in this
direction. However this may be, in my view the most practicable,
indeed the almost necessary, working attitude is to treat the chromo-
somes as if they were actually persistent individuals. The facts in
Metapodius, which at first sight seem to present so chaotic an
aspect, fall at once into order and become intelligible if regarded as
due to the presence in the species of a certain number of erratic
chromosomes, one or more of which may be introduced into the
zygote at the timeof fertilization and which in some sense retain their
identity throughout the development. The particular combina-
tion established at the time of fertilization is the result of the chance
union of two particular gamete combinations. Since the distribu-
tion of the supernumeraries to the spermatid nuclei is variable,
different gamete combinations occur in the spermatozoa of the
same individual; and the same is probably true of the eggs. More-
over, adults of the same species live side by side on the same food-
plants and presumably may breed together. Different combinations
may thus be produced in the offspring of a single pair, whether
the parents possess the same or different numbers. Metapodius
thus fulfills the prediction of Boveri, written nearly twenty years
ago. "Wenn bei einer Spezies einmalsehr viele und verschieden-
14 A view similar to this is suggested by Fick himself in his earlier discussion ('05, p. 204), but it
does not reappear in his later one.
198 Edmund B. Wilson
artige Irregularitaten vorkamen, diese sich wohl auf lange hinaus
erhalten miissten, so dass unter Umstanden Falle mit ausserordent-
lich grosser Variabilitat der Chromosomenzahl zur Beobachtung
kommen konnten, ohne dass selbst diese das Grundgesetz umstos-
sen vermochten, welches lautet: Es gehen aus jedem Kerngeriist
so viele Chromosomen hervor als in die Bildung derselben
eingegangen sind" ('90, p. 61). To the earlier expression of
this "Grundgesetz" Boveri has recently added the statement
that the chromosomes that emerge from the nucleus are not merely
of the same number but also show the same size-relations as those
that entered it. "Was durch den kurzen Ausdruck "Individuali-
tat der Chromosomen" bezeichnet werden soil, ist die Annahme
dass fur jedes Chromosoma, das in einen Kern eingegangen ist,
irgend eine Art von Einheit im ruhenden Kern erhalt, welche der
Grund ist, dass aus diesem ruhenden Kern wieder genau ebenso
viele Chromosomen hervorgehen und dass dieses Chromosomen
iiberdies da, wo vorher verschiedene Grossen unterschieden waren,
wieder in den gleichen Grossenverhaltnissen auftreten" ('07, p. 229).
The facts seen in Metapodius and other insects are thoroughly
in accord with the foregoing statement, and justify the additional
one that the chromosomes conform to the same principle in respect
to their characteristic modes of behavior. In the Hemiptera
heteroptera generally the idiochromosomes and supernumeraries,
the ra-chromosomes, and the "ordinary chromosomes" or "auto-
somes" show each certain constant peculiarities in respect to the
time of synapsis and behavior during the growth-period, and
assume a characteristic (though not entirely constant) mode of
grouping in the first spermatocyte. Perhaps the most obvious
of these facts is the very early condensation of the idiochromo-
somes and supernumeraries in the growth-period as contrasted with
the other chromosomes; and in the case of Pyrrochoris I have
shown ('09) that the idiochromosome never assumes a diffuse con-
dition after the last spermatogonial division. But even more
significant are the definite differences shown in the couplings of the
various forms of chromosomes that take place in the course of
the spermatogenesis. Nothing in these phenomena is more
striking than the accuracy with which these couplings take place.
Studies on Chromosomes 199
As Montgomery and Sutton have shown, the ordinary paired
chromosomes of the spermatogonia give rise to bivalents of corre-
sponding size at the time of general synapsis. The actual coupling
of the ordinary chromosomes at this time is still a matter of
dispute; 15 but no doubt can exist in regard to the couplings
that occur at a later period in case of the ra-chromosomes, the
idiochromosomes, and the supernumeraries. These characteristic
couplings are not determined merely by the size of the chromo-
somes. The union of the unequal idiochromosomes after the
second division takes place with the same regularity as that of the
equal ra-chromosomes in the prophases of the first. A small
supernumerary that is indistinguishable from the ra-chromosomes
in the spermatogonia never couples with the latter in either divi-
sion, but with the much larger idiochromosomes. The couplings
are equally independent of the original positions of these chromo-
somes, either in the spermatogonia or in the growth-period, as is
seen with especial clearness in case of the m-chromosomes. These
phenomena naturally suggest the conclusion that the couplings
result from definite affinities among the chromosomes. The possi-
bility no doubt exists that the couplings are produced by extrinsic
causes (such as the achromatic structures) but the evidence seems
on the whole opposed to such a conclusion. I consider it more
probable that they are due to intrinsic qualities of the chromosomes
and that the differences of behavior shown by different forms may
probably be regarded as due to corresponding physico-chemical
differences. This conclusion is in harmony with Boveri's experi-
mental results, though based on wholly different data. While
it does not seem worth while to attempt its wider development
here, I may express the opinion that all the chromosomes may con-
sist in the main of the same material basis, differing only in respect
to certain constituents; and further that the degree of qualitative
difference may vary widely in different species.
Zoological Laboratory
Columbia University
August 10, 1908
14 See for example, Meves ('07, pp. 453-468) who, like O. Hertwig, Fick and others, rejects the theory
of " individuality."
20O Edmund B. Wilson
ADDENDUM
The probability in regard to the female groups of Syromastes,
expressed in the footnote at p. 192 was first stated in my preced-
ing paper ('09, p. 73 ) after a study of the male only. Since the
present paper was sent to press I have had opportunity to ex-
amine females of this form. The facts are exactly in ac-
cordance with my prediction, the female groups containing 24
chromosomes, while the male number is 22. It now seems clear,
however, that the two idiochromosomes of Syromastes do not
correspond respectively to the large and the small idiochromo-
some of Metapodius or Lygaeus but are equivalent, taken together,
to the large idiochromosome or to the odd chromosome of
Anasa, etc.
October 25, 1908.
WORKS REFERRED TO
BONNEVIE, K. '08 Chromosomenstudien. I. Arch. f. Zellforschung, i, 23.
BOVERI, Th. '90 Zellenstudien. III. Ueber das Verhalten der chromatischen
Kernsubstanz bei der Bildung der Richtungskorper und bei der Be-
fruchtung. Jena, 1890.
'07 Zellenstudien. VI. Die Entwicklung dispermer Seeigel-Eier, etc.
Jena, 1907.
FICK, R. '05 Betrachtungen iiber die Chromosomen, ihre Individuality, Reduc-
tion, und Vererbung. Arch. Anat. u. Phys., Anat. Abth., Suppl. 1905.
'07 Vererbungsfragen, Reduktions-und Chromosomenhypothesen, Bas-
tard-Regeln. Merkel und Bonnet's Ergebnisse, xvi, 1906.
FOOT, K. AND STROBELL, E. C. '073 The "Accessory Chromosome" of Anasa
tristis. Biol. Bull., xii.
'o7b A Study of Chromosomes in the Spermatogensis of Anasa tristis.
Am. Journ. Anat., vii, 2.
GROSS, J. '04 Die Spermatogenese von Syromastes marginatus. Zool. Jahrb.,
Anat. Ontog., xii.
LEFEVRE, G.AND McGiLL, C. '08 The Chromosomes of Anasa tristis and Anax
junius. Am. Journ. Anat., vii. 4.
MOENKHAUS, W. S. '04 The Development of the Hybrids between Fundulus
heteroclitus and Menidia notata, etc. Am. Journ. Anat., iii.
Studies on Chromosomes 2OI
MEVES, Fr. Die Spermatocytenteilungen bei der Honigbiene, etc. Arch. Mikr.
Anat., Ixx.
MONTGOMERY, T. H. '01 A Study of the Germ-cells of Metazoa. Trans. Am.
Phil. Soc., xx.
'06 Chromosomes in the Spermatogenesis of the Hemiptera Heteroptera.
Ibid., xxi, 3.
PAULMIER, F. C. '99 The Spermatogenesis of Anasa tristis. Journ. Morph., xv,
Suppl.
SCHREINER, K. E. AND A. 'o8 Gibt es eine parallele Konjugation der Chromo-
somen ? Videnskebs-Selskabets Skrifter. i. Math-Naturw. Klasse,
1908, no. 4.
STEVENS, N. M. '06 Studies in Spermatogensis. II. A Comparative Study of
the Heterochromosomes in Certain Species of Coleoptera, Hemiptera
and Lepidoptera, etc. Carnegie Institution. Pub. 36, ii.
'o8a A Study of the Germ-cells of Certain Diptera, etc. Journ. Exp.
Zool., v. iii.
'o8b The Chromosomes in Diabrotica vittata, etc. Ibid., v, iv.
STRASBURGER, E. '08 Chromosomenzahlen, Plasmastrukturen,Vererbungstrager
und Reduktionsteilung. Jahr. wiss. Bot., xlv, iv.
WILSON, E. B. '053 Studies on Chromosomes, I. Journ. Exp. Zool., ii.
'o5b Studies, etc. II. Ibid., ii, iv.
'06 Studies, etc., III. Ibid., iii, I.
'09 Studies, etc., IV Ibid., vi, I.
'073 Note on the Chromosome-groups of Metapodius and Banasa. Biol.
Bull., xii, 5.
'07!) The Supernumerary Chromosomes of Hemiptera. Report of May
Meeting. N. Y. Acad. Sci. Science, n. s., xxvi, 677.
'o7c The Case of Anasa tristis. Science, n. s., xxv, 631.
202
Edmund B. Wilson
APPENDIX
List of individuals examined, arranged according to locality
No.
Species
Sex
Locality
Supernumeraries
Somatic
No.
^o. in
first
div.
i
terminalis
d 1
Madison, N. J. (Paulmier)
I small
2 3
13
2
terminalis
ff
Madison, N. J. (Paulmier)
I small
2 3
13
3-1 1
terminalis
d*
West Chester, Pa. (Montgomery)
absent
21
n
12
terminalis
if
West Chester, Pa. (Wilson)
absent
22
12
'3
terminalis
d 1
West Chester, Pa. (Wilson)
i large
2 3
!3
14
terminalis
9
West Chester, Pa. (Wilson)
i large
2 3
15
terminalis
9
West Chester, Pa. (Wilson)
2 large
24
16
terminalis
9
West Chester, Pa. (Wilson)
2 large
24
i?
terminalis
cT
Mansfield, Ohio
absent
22
12
if
terminalis
9
Mansfield, Ohio
absent
22
9
terminalis
d 1
Raleigh, N. C.
absent
22
12
20
terminalis
d 1
Raleigh, N. C.
i large
23
13
21
terminalis
(f
Raleigh, N.C.
2 large
2 4
14
22
terminalis
*
Raleigh, N. C.
I large, I small
24
H
2 3
terminalis
9
Raleigh, N. C.
absent
22
2 4
terminalis
9
Raleigh, N. C.
absent
22
2 S
terminalis
9
Raleigh, N. C.
i large
23
26
terminalis
9
Raleigh, N. C.
2 large
2 4
2?
terminalis
9
Raleigh, N. C.
2 large, I small
2 5
28
femoratus
d 1
Raleigh, N. C.
absent
22
12
2 9
femoratus
d 1
Raleigh, N. C.
absent
22
12
3
femoratus
9
Raleigh, N. C.
f large
23
3 1
femoratus
9
Raleigh, N. C.
2 large
24
3 2
femoratus
9
Raleigh, N. C.
4 lar g e
26
33
femoratus
9
Raleigh, N. C.
3 lar S e
2-3 small
2 7 -8
34
terminalis
c?
Southern Pines, N. C.
3 lar e
2 5
15
35
terminalis
d 1
Southern Pines, N. C.
3 large
2 S
IS
36
terminalis
d 1
Southern Pines, N. C.
4 lar e
26
ft
37
terminalis
c?
Southern Pines, N. C.
2 large
24
H
38
terminalis
d 1
Southern Pines, N. C.
3 large
25
39
femoratus
d 1
Southern Pines, N. C.
2 large
24
H
40
femoratus
d 1
Southern Pines, N. C.
2 large, 2 small
26
16
4i
femoratus
9
Southern Pines, N. C.
i large
n
42
femoratus
d 1
Columbia, S. C.
4 lar g e
26
16
43
terminalis
d 1
Charleston, S. C.
i small
2 3
J 3
44
terminalis
9
Charleston, S. C.
absent
22
Studies on Chromosomes
203
List of individuals examined, arranged according to locality Continued
No.
Species
Sex
Locality
Supernumaries
Somatic
No.
No. in
first
div.
45
femoratus
9
Charleston, S. C.
2 large 24
46
femoratus
d"
Savannah, Ga.
absent
22
12
47
granulosus
c?
Tucson, Arizona
absent
22
12
48
granulosus
.-
*
12
13
16
17
. . 29
The Journal of Experimental Zoology, Vol. VI. . .
STUDIES ON CHROMOSOMES
VI. A NEW TYPE OF CHROMOSOME COMBINATION
IN METAPODIUS
RETURN TO
DIVISION OF GENETICS
HILGARO HALL
Ki)\ir\i) H. WILSON
WITH FI\ I
REPRINTED FKOM
THE JOURNAL OF EXPERIMENTAL ZOOLOGY
Volume IX No. 1
\Vn,!,IAMS .fe SVILKIXS COMI'ANV
BALTIMORE
Reprinted from THE JOURNAL OF EXPEIKMENTAL ZOOLOGY VOL. 9, No. 1.
STUDIES ON CHROMOSOMES
VI. A NEW TYPE OF CHROMOSOME COMBINATION IN METAPODITJS
EDMUND B. WILSON
Professor of Zoology, Columbia University.
WITH FIVE FIGURES
Although the peculiar combination of chromosomes here to be
described has been seen in only a single individual, it affords new
and I think significant evidence regarding some of the most inter-
esting of the problems connected with the nuclear organization.
As was shown in the fifth of my "Studies on Chromosomes," 1
the genus Metapodius is most exceptional and remarkable in
that the specific number of chromosomes varies, while that of the
individual is on the whole constant. It is true that slight indis-
criminate fluctuations in the number of the ordinary chromosomes,
or "autosomes, " occur, as they do in many other species; but
this is only an inconsiderable source of the specific variation. The
evidence shows, beyond a doubt in some individuals, and hence
with probability for all, that the numerical differences are pri-
marily due to variations in the number of a particular class of
chromosomes which I called the "supernumeraries." These
may be wholly absent. When present, their number is constant
in the individual, but differs in different individuals. They are
often recognizable in both sexes by their size, and in the male
also by certain very definite peculiarities of behavior in the matu-
ration-process. When they are absent, the diploid groups contain
22 chromosomes; and this condition is almost certainly the funda-
mental type of the genus, of which all the other conditions are
variants. Such a group comprises 18 ordinary chromosomes, or
"autosomes" + 2 very small microchromosomes, or m-chrom-
somes + 2 unequal idiochromosomes = 22 (these respective
1 Wilson: '09c.
THE JOURNAL OP EXPERIMENTAL ZOOLOGY VOL. 9, NO. 1.
54 EDMUND B. WILSON
classes having the peculiarities heretofore described). 2 Numbers
above 22 arise through the addition of one or more relatively
small "supernumeraries," which agree in behavior with the small
idiochromosome, of which they are probably duplicates. None
of my own material (53 individuals, of three species) showed less
than 22 chromosomes, and at least one small idiochromosome was
present in all. In all of Montgomery's material of M. terminalis,
however (9 individuals), this chromosome is absent, the sperma-
togonial number is but 21, and the large idiochromosome appears
without a synaptic mate as a typical odd or accessory chromosome.
The foregoing results were based on the study of 62 individuals
in all, representing the three species, terminalis, femoratus and
granulosus. In February, 1909, 1 took at Miami, Fla., two addi-
tional male specimens of femoratus, quite typical in structure,
and closely similar in external appearance. One of these (No. 63)
is an ordinary 23-chromosome form with one large supernumerary
(like Nos. 13 or 48 of the general list given in " Study V") and is
only of interest for comparison with the other individual. The
latter, hereinafter designated as "No. 64," shows a different
chromosome-combination from any heretofore seen in this genus
or elsewhere. The diploid groups (spermatogonia) contain 22
chromosomes; but both these groups in themselves and their
history in maturation proves most clearly that they are not the
same as in the typical 22-chromosome forms, differing from the
latter in respect to the idiochromosom.es and the m-chromosomes.
In the typical forms there are, as stated above, two of each of
these chromosomes. In No. 64, on the other hand, there are three
m-chromosomes and but one idiochromosome (the large), the
latter appearing as a typical odd or accessory chromosome, as in
the material of Montgomery; thus, 18 autosomes + 3 m-chromo-
somes + 1 odd chromosome = 22. That this is the true interpre-
tation of the facts is demonstrated by the behavior of these respec-
tive chromosomes in the maturation-process. I would emphasize
the fortunate fact that both testes of the animal show excellent
fixation and staining (strong Flemming, iron haematoxylin) and
that they contain multitudes of division-figures which demonstrate
* Ibid: '056, 05c, '06, etc.
STUDIES ON CHROMOSOMES 56
all the stages. The agreement of great numbers of division-fig--
ures from both testes leaves no doubt regarding the constancy of
the essential phenomena (with rare minor variations, as indicated
beyond). As will be seen, the modification of the diploid groups
has led to corresponding modifications of the maturation-process
that are most interesting in relation to some of the problems of
synapsis and of the qualitative differences of the chromosomes.
DESCRIPTIVE
a. The spermatogonial groups
The peculiar anomaly of the chromosome groups, first seen in
the spermatocyte-divisions, led me to examine the spermatogonial
groups with particular care, and it will be worth while to state
both the preliminary and the definitive results. These groups
are in the nature of the case more difficult than those of the sper-
matocytes, owing to the greater number, smaller size, and greater
crowding of the chromosomes; hence, only flat metaphase-plates
and such as are not very oblique to the plane of section can safety
be used. A search through the numerous dividing spermatogonia
showed 35 cases that seemed to meet these conditions and also
to show no serious obscurity or confusion of the chromosomes.
Many of these are of almost schematic clearness, and some are
well adapted for photographic reproduction. The first examina-
tion showed undoubtedly that 29 of the 35 cases contained 22
chromosomes each, including 19 large and three very small ones.
Of the six exceptions, three seemed to lack one of the small ones,
two, one of the large ones, and one a large and a small. Closer
study of these six cases ultimately showed that in four cases
the apparently missing third small chromosome was in reality
present, though hidden among the larger chromosomes, while in
two cases an apparently missing larger chromosome was found
lying immediately below another one, the metaphase-plate not
yet having become perfectly flat. This leaves but one exception
in 35 cases, and we shall hardly go astray in the conclusion that
this exception is probably the result of accident. In any case we
may confidently conclude that the chromosome-group shown in
56 EDMUND B. WILSON
the 34 cases may be taken as characteristic of the dividing sper-
matogonia, and that it occurs with a high degree of constancy.
Six of these groups, from the best that could be found, three
from each testis, are shown in fig. i, a-/. These have been se-
lected particularly to show the different positions of the three
small chromosomes. The latter appear to follow no rule what-
ever, the three lying anywhere in the metaphase-plate; and all
may be separate, all together, or two together and one separate.
This is an interesting and significant fact, because in the first
spermatocyte-division, as described beyond, the three are always
associated to form a triad element which invariably occupies the
same position in the chromosome-group (see p. 58).
For the sake of comparison, four sperm atogonial groups from
other individuals are here reproduced (from my fifth Study).
Two of these (fig. i, i, j] are from femoratus, No. 29, which has
the typical diploid group of 22 chromosomes, including but two
small (m-chromosomes.) The other two (fig. i, g, h) are from
terminalis, No. 2, which has 23 chromosomes, including two m-
chromosomes and one small supernumerary 3 . As will appear
beyond, this latter chromosome is wholly different in nature from
the third small chromosome in individual No. 64, though indis-
tinguishable from it by the eye in the spermatogonial groups.
As the figures show, the larger chromosomes in No. 64 show
well marked size-differences, and in most of the groups a largest
and second largest pair are usually fairly evident; but it is impos-
sible to pair all of the chromosomes accurately by the eye. It
is, however, obvious that not more than 18 of the 19 can be equally
paired. One of them must either have no proper mate, or it must
form a very unequal pair with the third small chromosome. The
following possibilities must, accordingly, be considered:
1. The nineteenth large chromosome and the third small one
are respectively a large and an abnormally small idiochromosome
which form a pair of synaptic mates, or
2. The nineteenth large chromosome is an odd or accessory
chromosome, without a synaptic mate, while the third small one
is similar to a small "supernumerary" or
* Cf: Photo. 29, Study V.
STUDIES ON CHROMOSOMES
57
All the figures are from camera lucida drawings. With a few exceptions they
are a little more enlarged than those of Study V.
o
Fig. 1 a-f, spermatogonial groups, M. femoratus, No. 64, three from each tes-
tis; g, h, spermatogonial groups for comparison from M. terminalis, No. 2,
with one small supernumerary (23 chromosomes); i, j, spermatogonial groups
from M. femoratus, No. 29 (22 chromosomes); k, I, early prophases, No. 64; m,
n, late prophase of same ; o, late prophase for comparison, from M. terminalis, No.
43, with one small supernumerary (23 chromosomes).
58 EDMUND B. WILSON
3. An odd or accessory chromosome is present, and also a
third w-chromosome.
A study of the maturation process decisively establishes the
third of these possibilities as the fact.
b. The first spermatocyte-division
Both testes contain immense numbers of both spermatocyte-
divisions in all stages, and many of the cysts show the facts with
great beauty. The first division itself at once indicates the true
interpretation of the spermatogonial groups; and this is consis-
ently borne out by the stages which precede and follow.
In polar views (fig. 2, d-g] the first division metaphase is iden-
tical in appearance with that of Anasa, Chelinidea, Narnia, and
other coreids that have 21 spermatogonial chromosomes (including
Montgomery's individuals of M. terminalis). Eleven chromo-
somes appear, including one very small central one surrounded
by a ring of nine much larger ones, while the eleventh usually
occupies a position outside the ring, as in figs. 2d, 2f, (figs. 2e, 2g
are given to show exceptions to this). From these views alone
we should infer that the spermatogonial number is 21, that the
small central chromosome is the m-bivalent, and that the eccentric
one is the accessory. This will appear upon comparison with
figs. 2, h, i, which show two corresponding views of Montgomery's
material of M. terminalis. Side views at once reveal the fact,
however, that the central body in No. 64 is not a bivalent but a
triad element, consisting of three small chromosomes united end
to end (figs. 2b, 3a, &,) and it is perfectly evident that these are
identical with the three very small ones of the spermatogonial
groups. Hundreds of these figures have been observed, iri almost
all of which the three components have the linear arrangement
just described; but now and then a different grouping occurs, as
may be seen in both side (fig. 3c) and polar views (fig. 2g).
In the ensuing division the ten larger chromosomes divide
equally, showing as they draw apart the curious forms represented
in fig. 4, which are closely similar to those described in Anasa
by Paulmier ('99). As the figures show, the chromosomes in
STUDIES ON CHROMOSOMES
59
d
4J* A
% * w *-* , *
'
>6
I
;
>i'
Fig. 2 a, late prophaseNo. 64, spindle forming; b, metaphase of same in side
view; c, late prophase of M. femoratus, No. 29 (22 chromosomes), for comparison
with a; d-g, first division metaphase, polar views, No. 64; h, i, similar views of
M. terminalis, No. 3 (Montgomery's material), with 21 chromosomes;./, similar
view of M. femoratus, No. 29 (22 chromosomes); k, similar view of M. terminalis,
No. 43 (23 chromosomes), with one small supernumerary, for comparison.
the early anaphase are more or less clearly quadripartite, and sep-
arate into bipartite daughter chromosomes connected by conspic-
uous double fibers (figs. 4, b-h) but true tetrads (such for instance,
as those observed by Levfere and McGill in Anax, '08) are rarely
if ever seen. The quadripartite form, though very characteristic
of this division, is by no means invariable in case of the large
bivalents, and has not been seen in case of the eccentric odd
chromosome.
60 EDMUND B. WILSON
In the mean time the small central triad breaks up into its
separate components, which then pass to the poles in a very inter-
esting fashion. This process always begins before the division of
the large chromosomes, and is subject to some variation. Most
frequently the three components draw apart in such a way as to
leave the middle one lagging near the equator of the spindle while
the others are proceeding towards the poles (flgs. 3/i, i). Often,
however, one component first separates from the other two (figs.
3j, k) ; but even in this case it seems probable that one of the latter
is afterwards left lagging on the spindle, since later in the anaphases
this arrangement is almost invariable. In these stages the middle
component frequently becomes drawn out along the spindle to
form a rod which finally passes to one pole to enter the telophase
group (figs. 4e,/) . Half the secondary spermatocy tes thus receive
two small chromosomes and half but one, the respective numbers
being 12 and 11.
Two observed anomalies may briefly be mentioned. In two or
three cases the middle component seems to be degenerating on
the spindle (fig. 40) ; but if this be really the case it must be of
rare occurrence, as is shown by the second division. Another
interesting anomaly is shown in fig. 4/i. Here there are appar-
ently five small chromosomes, two of which are smaller than the
others and are connected by a fiber as if they had recently divided.
I am uncertain how to interpret this case, for one of the larger
chromosomes (stippled in the figure) is paler than the others and
lies at a lower level. This may be a fragment of the original
plasmosome. If this be the case we have before us a case in which
the central small chromosome has divided precociously. If all
the five bodies, on the other hand be chromosomes, one of them
would seem to be an extra or adventitious body, comparable to
those described and figured by Paulmier in Anasa ('99, fig. 28a).
c. The second spermatocy te-division
As is to be expected from the asymmetrical distribution of the
three small chromosomes in the first division the secondary sper-
matocvtes are of two classes. These divisions are very numerous
STUDIES ON CHROMOSOMES
61
Fig. 3 Metaphases and anaphases of the first division, No. 64, in side view, a,
b, typical side views, with linear central m-triad; c, unusual grouping; d, similar
view of M. terminalis, No. 1, for comparison, with one small supernumerary and
m-bivalent (23 chromosomes) ; e. g, similar views of M. femoratus, No. 29 (22 chro-
mosomes) for comparison;/, the same, M. femoratus, No. 46 (22 chromosomes) ;
h-k, No. 64, initial anaphase, separation of the m-triad.
in both testes, and all the stages are shown by hundreds. In
polar views of the metaphases about half the cells are seen to
contain 11 chromosomes (fig. 5a, 6) and half 12 (fig. 5c, d), the
former containing but one small chromosome and the latter two.
62
EDMUND B. WILSON
Fig. 4 Anaphases of first division, all from No. 64; g and h are atypical condi-
tions.
As is the rule throughout the Coreidse, the regular grouping char-
acteristic of the first division is usually lost or obscured in the
second. As a rule the ring formation is no longer seen, there is,
no constantly eccentric chromosome, while the w-chromosome,
invariably central in position in the first division, now occupies
any position, though* it is more frequently near the center of the
group.
In side views of the metaphases all of these chromosomes, with
one important exception, are dumb-bell shaped, and in the initial
anaphases are seen drawing apart into a pair of daughter-chromo-
somes (fig. 5e-gr). One chromosome, almost invariably central
in position, forms an exception in showing no sign of constriction,
its form being evenly rounded and often nearly spheroidal. As
a
fST \
, m
I m
p
III!
Fig. 5 Second division, No. 64. a, 6, metaphases, polar views, 11 chromo-
somes; c, d, the same, 12 chromosomes; e, side view; /, the same showing all the
chromosomes (four from lower level shown below) ; g, initial anaphase, all the
chromosomes shown (five of them from a lower level at right) ; h-o, later anaphases;
p, q, sister groups, from the same spindle, late anaphase, p, the upper group with
11 chromosomes; q, the lower group with 12; r, s, two late anaphase groups (not
from the same spindle) to show the third and fourth types of spermatid nuclei.
64 EDMUND B. WILSON
seen in side views of the late metaphases or earliest anaphases
(fig. 50) this chromosome always appears darker and more con-
spicuous than the others (probably because it is not drawn out
along the spindle fibers) and owing to this circumstance its history
during these stages may be followed with an ease and certainty
of which the figures give but an imperfect idea. As the bipartite
chromosomes separate in the anaphases the chromsome in ques-
tion is usually left lagging near the equator of the spindle, though
not infrequently it lies in one of the daughter groups (figs. 5, h-ri) .
In the late anaphases, as the cell-body is dividing, it may be seen
passing, without constriction, diminution in size, or other sign
of division, into one of the daughter-nuclei (fig. 50). I desire
especially to emphasize the fact that these processes are seen
with such clearness and in so great a number of cells, as to leave
not the remotest 'doubt that this chromosome neither divides
nor separates from an accompanying mate. It is therefore a
typical odd or accessory chromosome, or unpaired idiochromo-
some, identical in its general history with that seen in Anasa,
Protenor, and so many other forms. In the second maturation-
division, accordingly, one of the daughter-cells in each case re-
ceives one chromosome less than the other; and since there are
two classes of secondary spermatocytes, there are four classes of
spermatids and of spermatozoa. All receive nine ordinary
chromosomes and one m-chromosome. Two-fourths contain and
two-fourths lack a second small chromosome ; and each of these two-
fourths falls into two classes, one containing and one lacking the
accessory chromosome. These four classes are readily distinguish-
able in polar views of rather late anaphases, particularly in cases
where the accessory chromosome lies at the same level as the chro-
mosomes of one daughter group. Fig. 5p,q show two such daugh-
ter groups, from the same spindle and in the same section. In
each case two small chromosomes are present, and one group
contains 11 chromosomes, the other 12. I could not find a single
case of the other type (with 10 and 11 chromosomes) in which
both daughter-groups appear in the same spindle: but two ana-
phase groups from different spindles are shown in fig. 5r, s, the
former containing 11 chromosomes, the latter 10.
STUDIES ON CHROMOSOMES 65
d. The Growth-period and Maturation-prophases
The foregoing facts demonstrate in the clearest manner that
this individual of M. femoratus differs from all other individuals
of the genus heretofore examined, with the exception of Mont-
gomery's material of M. terminalis, in having an odd or unpaired
idiochromosome (accessory chromosome) which corresponds to
the larger member of the pair of unequal idiochromosomes found
in other individuals. They show also that the third small chro-
mosome is not a small supernumerary of the -type found in other
individuals, and is nothing other than a third m-chromosome.
This is fully borne out by the growth-period and prophases. As I
have indicated in earlier papers, the m-chromosomes are in general
characterized during the growth-period by the fact that they re-
main univalent (there are some exceptions to this) and in most
cases (of which Metapodius is one) are in a diffuse and light-
staining condition. Further, as was first shown by Gross ('04) in
Syromastes, as a rule they only conjugate to form a bivalent in
the final prophases of the first division very often not until the
spindle is formed and the chromosomes are entering the equator-
ial plate. Such a late prophase, from a 22-chromosome individual
of the same species (No. 29) is shown for the sake of comparison
in fig. 2c, the two separate m-chromosomes appearing above
and towards the left. Their final conjugation always takes place
at the center of the group (fig. 2j, 3, e-g).
In individual No. 64, prophases of every stage are shown in
hundreds of nuclei. In the latest stages, after the nuclear wall
has broken down, three separate small chromosomes are shown
(fig. 2a) which may be seen coming together in the final prophases
to form the small central triad. Figs. 1m, n show two earlier
stages from the same cyst with the last, one of them showing the
beginning of the spindle-formation, the other an earlier stage
when the asters are very small and often invisible. Each of these
shows the three separate small chromosomes, as before. At this
time all the chromosomes are compact and deeply stained. In
still earlier stages, at a time when the bivalents are all diffuse and
appear in the form of lightly staining double crosses, rods, etc.,
THE JOURNAL OP EXPERIMENTAL. ZOOLOGY, VOL. 9, NO. 1.
66 EDMUND B. WILSON
the three small chromosomes are still easily seen in many of the
nuclei; but they are now pale and diffuse like the bivalents. In
this respect the third small chromosome differs from a " super-
numerary" of the type described in my former paper, and agrees
exactly with the m-chromosomes.
Each nucleus contains at this period a single compact, rounded
and* intensely staining chromatic nucleolus, which is no doubt
the odd or accessory chromosome (monosome), as in so many
other forms, 4 and in addition there is present a conspicuous, rounded
4 This identification is in agreement with that of most observers in recent years.
A few writers have however disputed the view that the chromatic nucleolus of
the growth period of the spermatocytes is a chromosome e. g., Moore and
Robinson in case of the cockroach ('05), Foot and Strobell in the case of Anasa('07)
and Euschistus ('09), and Arnold ('08), in case of Hydrophilus. The results of
Moore and Robinson on this point are opposed by those of Stevens ('05),Wassilieff
('07), and more particularly by the detailed observations of Morse ('09). Those
of Foot and Strobell on Anasa are not sustained by the later ones of Lefevre and
McGill ('08). Among others who have in the past two years adhered to the view
here adopted may be mentioned Otte ('07), Davis ('08), Boring ('07), Jordan ('08),
Stevens ('08, '09), McClung ('08), Robertson ('08), Randolph ('08), Nowlin ('08),
Payne ('09,) Wilson ('096, '09c), Gutherz ('09), Wallace ('09), Gerard ('09), and
Buchner ('09a). Since I intend to return to the subject hereafter I will take this
occasion for only brief comment on some of these results, without attempting a
full review of the literature.
Moore and Robinson, who have been followed by Arnold (Strasburger, '07, '09.
expresses the same opinion) also regard the body that is seen passing to one pole in
one of the maturation divisions ("accessory chromosome") as not a chromosome
but a ' 'nucleolus." I find it incredible that anyone can hold to such a view who
reckons squarely with the large existing body of direct and detailed observation
upon the accessory chromosome itself; and this view seems to be quite ruled out of
court by comparative studies on the sex-chromosomes, such for instance as those
of Payne on Gelastocoris and the reduvioids. I will not enter here upon the maze
of difficulties regarding the numerical relations of the chromosomes which the
same view involves, since they have already been indicated by Gutherz ('09), in a
recent reply to Strasburger. My own preparations, including an extensive series
of sections and smears especially of Protenor, Lygaeus and Pyrrhocoris leave in
my mind not the least doubt of the identity of the chromatin-(chromosome)-
nucleolus of the growth period with the odd chromosome (monosome) of the
spermatogonia, and with the heterotropic or accessory chromosome of the mat-
uration-divisions.
Certain writers have seemed to take it for granted that the accessory chromosome
or "monosome" is always characterized by its nucleolus-like condition in the rest-
ing nuclei, not only in the spermatocytes but also in the spermatogonial and other
STUDIES ON CHROMOSOMES 67
pale plasmosome which is considerably larger than the chromo-
some nucleolus. This body, particularly well shown in these
slides, is at once recognizable by its smooth contour, spheroidal
form (sometimes double, as in fig. II) and pale yellowish color
after the hsematoxylin, and it forms a striking contrast to the
intense blue-black of the chromosome-nucleolus. Nuclei in
which all five bodies the three small chromosomes and both
diploid nuclei. This assumption which is doing much to confuse the whole sub-
ject may accord with the facts in certain species, but certainly is not generally
true. Much of the recent work in this field, as well as some of the earlier (e. g., that
of McClung '00, and Button '00) goes to show that in many species it is only in
the growth-period of the spermatocytes that this chromosome forms a chromosome
nucleolus, not in the diploid nuclei of either sex. Such seems to be the case in all
the Hemiptera that I have studied. In these animals the accessory chromosome,
or its homologue the large idiochromosome, first assumes the nucleolus-like con-
dition in the post-spermatogonial stages, when its origin from an elongate chromo-
some may in some species readily be followed step by step, as I have shown in
Lygaeus ('056) and Pyrrhocoris ('096). In the spermatogonia of these animals
this chromosome does not differ visibly in behavior from the others and cannot
be seen in the resting nuclei.
Several years ago, in two successive papers ('05a, '06) I described and commented
on the interesting fact that in the female this chromosome (and its fellow, when
present) seems in some species not to assume a nucleolus-like condition in the
synaptic stage and early growth-period of the oocytes. Since some doubts on
this point were raised in my own mind by the later work of Stevens ('06) and Gut-
herz('07)I am now glad to have the very positive confirmation of my results given
by the work of Foot and Strobell ('09) on Euschistus (one of several forms I had
examined). This confirmation must have been made without knowledge of my
previous work, since the latter is referred to in neither text nor literature list, and
the supposedly new facts are made the main basis for renewed attack upon my
general conclusions. On the other hand Buchner ('09a) has recently found in the
synaptic or "bouquet" stage of the oocytes in Gryllus a nucleolus-like "accessory
body" which he believes to be of the same nature as the accessory chromosome
of the male, though its history in maturation was not followed out, nor is other
proof of the conclusion given.
It is of some psychological interest to find Buchner on the one hand and Foot
and Strobell on the other disputing my conclusions regarding sex-production on
diametrically opposing grounds, the first-named author because (as he believes)
a chromosome-nucleolus is present in the oocyte-nucleus, the last named because it
is absent(\). In what way either of the mutally contradictory arguments inval-
idates or weakens my conclusions I am not yet able to perceive, nor need we here
consider the contradiction in the data; but it is interesting to observe how each
of the arguments goes awry by reason of the confusion regarding the chromosome-
nucleolus, referred to above. Foot and Strobell, for example, argue that because
68 EDMUND B. WILSON
nucleoli are visible in the same section are not very common.
Two such cases are shown in figs. Ik, L, each of which shows also
four of the nine larger bivalents.
I have not endeavored to make an exhaustive study of the
growth-period as a whole, but the facts reported above taken in
such a body is not present in the oocyte-nucleus, therefore the odd or accessor}'
chromosome of the male cannot be derived in fertilization from the egg-nucleus
an obvious non sequitur. Buchner's argument, based upon precisely opposite
data, shows a somewhat similar, though less obvious entanglement. The essence
of his objection is given in the following passage, which at the outset accepts
all the essential facts on which the conclusions of Stevens and myself were based.
"Auf alle Falle haben wir nur eine Sorte von Eiern, denn dasser (the accessory
chromosome) in einem Ei ausgestossen und im andern innenbehalten wird, er-
scheint undenkbar. Die Spermatozoen haben das accessorische Chromosom zur
Halfte. Nehmen wir an, die Eier besassen das accessorische Chromosom schon,
so gabe es Tiere mit zwei Monosomen und solche mit einem ein Fall der nicht
ezistiert" (op. cit., p. 409, italics mine). This is, indeed, an astounding statement;
for it was the very fact that there are individuals that have but one monosome
or accessory chromosome (the males), and other individuals of the same
species (the females) that have two corresponding chromosomes, upon which the
conclusions of Stevens and myself were mainly based (!). This is true, asGutherz
('08) has shown, of the very form (Gryllus) of which Buchner is writing, the single
odd chromosome (monosome) of the male, recognizable by its peculiar form and
other characters, being represented in the female by two such chromosomes.
This is also in agreement with the results of other recent workers on the Orthoptera
including Wassilieff, Davis, Jordan and Morse. I can therefore find no meaning
in Buchner's statement unless the word "Monosom" be used to denote simply
a chromosome-nucleolus, when the passage becomes at least intelligible. But
such a restriction in the meaning of this word is not justified by its etymology,
by the original definition of its author (Montgomery, '06a, '066) nor by the facts;
and it does not seem to accord even with Buchner's own usage elsewhere in the
paper. That Buchner's statement is totally at variance with the facts when cor-
rectly stated is shown by the following summary of my results, quoted from one
of the papers in which Montgomery first defined the word "monosome." "When
there is a single monosome in the spermatogenesis (as in Protenor, Harmostes,
Anasa and Alydus) there are two in the ovogenesis so that the ovogonia possess
always an even number of chromosomes" ('066, p. 145, italics mine).
But even if we admit that the "accessory body" of the female is a chromosome
and not only is there no proof of this but many reasons for doubting it what ad-
verse bearing would the fact have upon the "theory"? None as far as I can see,
unless this chromosome were proved to be univalent and without a synaptic mate
Were all this true,new and unintelligible complications would arise in regard to the
numerical relations of the diploid and haploid chromosome-groups in both sexes;
but it is not worth while to consider these puzzles since they lie in a region not of
observed fact but of pure phantasy.
STUDIES ON CHROMOSOMES 69
connection with the spermatocyte-divisions, are thoroughly deci-
sive in showing the third small chromosome to be an extra ra-chro-
mosome not distinguishable in any respect from the other two.
2 DISCUSSION
It seems to me that in the individual of Metapodius that has
here been described nature has performed an experiment which,
as far as it goes, is precisely such as we should like to carry out
artificially in order to test the hypothesis of the genetic continuity
of the chromosomes and the question of their qualitative relations
in the maturation-process. . The experiment (if we may call it
such) consisted in the omission from the typical 22-chromosome
diploid groups of the small idiochromosome, and its replacement
by one of different type, a third m-chromosome. In what way
this was effected can only be conjectured; but it seems altogether
probable that the anomaly was present in the original fertilized
egg, as a result of one preexisting in one or both the gamete-
nuclei. 5 In any case we may be sure that it arose very early in the
ontogeny, at a period prior to the separation of the right and left
gonads, since both testes show precisely the same characters.
It is certain that the initial anomaly has persisted unchanged
through many generations of cells, and that the alteration in the
diploid groups has involved corresponding modifications in the
maturation-process. The significant fact is that throughout this
process the chromosome that has been added does not take the place of
the one that has been omitted, but behaves according to its own kind.
This is a truly remarkable result when we consider that the num-
ber of chromosomes in the diploid groups (22) remains unaltered.
These groups still consist of 11 pairs of chromosomes; but one is
5 We must assume, in this case, that the sperm-nucleus contained no small idio-
chromosome and that either this nucleus or the egg-nucleus contained two m-chro-
mosomes. The former condition may have resulted from a failure of the idiochro-
mosomes to separate in the second spermatocyte-division (which, as I have shown,
may actually occur). The presence of a second w-chromosome may be due to a
similar cause.
70 EDMUND B. WILSON
an unnatural or hybrid pair, which consists of non-homologous
members the large idiochromosome and the third m-chromosome.
The facts show most decisively that these two chromosomes do
not play the part of synaptic mates towards each other, but retain
each its own characteristic behavior. In synapsis the third
m-chromosome invariably couples with its own kind to form a
triad element while the large idiochromosome remains unpaired.
Thus the substitution of one chromosome for another of a different
kind has been followed by no regulative process, and a perma-
nently new combination has been produced. The full force of this
conclusion first becomes evident when we compare the present case
with those in which there is present a single small supernumerary
of the type described in my fifth Study. In the diploid groups such
a supernumerary is quite indistinguishable from a third m-chro-
mosome as we may see, for instance, upon comparison with figs.
Ik, 7, t-y, photograph 29, of Study V. 6 In the first spermatocyte-
division also, in cases where a small supernumerary lies within the
ring of large bivalents (as in photograph 6, fig. 7i of my fifth Study)
side-views give a picture almost indistinguishable from such a
condition as that shown in fig. 3c. Such a side view of terminalis
No. 1, is given for comparison in fig. 3d, the two m-chromosomes
just separating at the center of the group, and the supernumerary
(s) just to the left. The resemblance between this figure and fig.
3c is so close as to amount almost to identity. It seems incredible
that the behavior of the third small chromosome in the ensuing
division should not be identical in the two cases ; and it should be
identical were the history of the individual chromosomes in matur-
ation determined merely by their size or their mechanical rela-
tion to the achromatic figure. In point of fact, however, the small
supernumerary and the third m-chromosome show characteristic
differences throughout the whole process. In the growth-period
the former appears as a condensed chromosome-nucleolus, usually
coupled with the idiochromosome-nucleolus, while the m-chro-
mosome remains diffuse and usually free. In the first division
the former divides as a univalent (i. e., it is typically uncoupled,
though it may be in contact with the idiochromosomes) and is
8 Loc. cit.:'09c.
STUDIES ON CHROMOSOMES 71
usually outside the ring (as in fig. 2h) ; while the third ra-chromosome
is always coupled with the two others at the center of the ring, and
moves to one pole without division. In the second division these
relations are almost exactly reversed, the ra-chromosome dividing
equationally as a univalent, while the supernumerary does not
divide and is typically coupled with the idiochromosome bivalent
near the center of the group. I desire to emphasize the fact that
these differences are in no way obscure or difficult to see, but are
conspicuously shown in so great a number of cells as to remove
all doubt. 7
7 This point demands emphasis because of the scepticism expressed by certain
writers in regard to the constancy of the chromosomes in respect to number, size
and behavior. Conspicuous among these writers is Delia Valle ('09) who has
brought together a valuable if somewhat uncritical review of the literature, and
contributes careful observations of his own upon variations in the chromosome-
number in the somatic cells of Salamandra. Such scepticism is perhaps not sur-
prising in view of the unlucky contradictions that still exist in the literature even
of so favorable and well known a group as the insects. But to ascribe this con-
fusion of the literature to a confusion of the facts i. e., to an inconstancy so great
as to preclude the possibility of attaining exact results would be, I think, a fatal
blunder. The confusion in the literature cannot, of course, be attributed altogether
to mistakes of observation or to accidents of technique though both these must
be held to a strict reckoning. I am not aware that anyone has maintained that
the relations of the chromosomes form an exception to all other biological phenom-
ena in being absolutely fixed and immutable; and due weight should be given
to the numerical variations that have been recorded by Delia Valle and many others
myself included. The fact remains that it is possible to determine accurately
what are the normal or typical relations of the chromosomes, as of other struc-
tures, and to establish in many cases their high degree of constancy. The same
common sense must be used in the treatment of these relations as in the case of
other phenomena that are subject to variation. For example, insects have been
seen with seven legs, but it is not for this reason to be doubted that insects have
six legs. In like manner, in the ovaries of Largus cinctus I have seen as many as
three dividing cells that show 13 chromosomes; but I nevertheless do not doubt,
after the study of a large number of cases, that the typical number is 12.
The case of Metapodius is disposed of by Delia Valle in the following easy fash-
ion. "Not constancy but variability in the number of chromosomes is the general
rule in all organisms; of which the observations published by him (Wilson) are
but a special confirmatory case" (op. cit., p. 161, translation). Better acquaintance
with the facts in Metapodius would probably render Prof. Delia Valle less certain
of this; for I am confident that no observer of ordinary competence could confuse
such a series of relations as that here displayed with the occasional fluctuations
with which we are familiar in many forms, including this very genus.
72 EDMUND B. WILSON
It seems to me that such facts have the value of actual experi-
mental evidence in support of the hypothesis of the genetic con-
tinuity of the chromosomes and that of their qualitative difference.
All will admit that the peculiarities of the later generations of cells
in this individual of Metapodius are inherited from earlier ones.
It is the obvious, natural and, I think, inevitable conclusion that
the third ra-chromosome, introduced at an early period, has not
lost its identity in the later stages. If its presence is merely owing
to a corresponding excess of chromatin, how shall we account for
the characteristic peculiarities of behavior that differentiate it
so sharply from an ordinary " supernumerary " of corresponding
size? To reply that the excess represents a particular kind of
chromatin that is re-segregated at each division in the form of a
particular chromosome is to grant the most vital assumption
in the hypothesis of genetic continuity.
I think that sufficient emphasis has not yet been laid upon the
support given to this hypothesis by the variable position of the
chromosomes in the diploid groups. I have several times pointed
out in this paper and preceding ones, that there is no constancy in
the relative position of the spermatogonial chromosomes as may
be seen with particular clearness in case of the ra-chromosomes of
the Coreidse or the small idiochromosome of the Pentatomidae or
Lygseidse, or of chromosomes distinguishable by their large size,
such as are seen in Protenor, Largus or Anasa. This is certainly
not what we should expect were the chromosomes merely " tactic"
formations that appear in characteristic array, as a crystal form
in a solution, merely because of the specific properties of a single
chromatin-substance as such. Two answers might be made to
this. It might be said that the chromosomes merely represent
the segregation of so many different kinds of chromatin that are
mixed together in the resting nucleus. 8 I am disposed to regard
this as a tenable hypothesis; but obviously it grants the most essen-
tial part of the continuity hypothesis. Again it might be said that
the chromosomes are originally formed always in the same j osi-
tion but lose it by subsequent shiftings in the prophases. It
Cf. Fick: '05; Wilson: '09c.
STUDIES ON CHROMOSOMES 73
would be difficult to disprove this in ordinary cases; but fortun-
ately Boveri's studies on Ascaris ('09) have shown beyond all doubt
that in this form there is no constancy in the original position of
the prophase chromosomes, the only definite order being shown in
the close agreement of each pair of daughter-cells. The position
of the prophase chromosomes as Boveri shows with great cogency
is here a consequence of the position in which they entered the nu-
cleus in the preceding telophases ; as the latter position is itself due
to causes (which may be quite fortuitous) that determine the posi-
tion of the preceding metaphase chromosomes.
The facts support no less directly and strongly the conclusion
that the chromosomes differ among themselves in a definite way
in respect to their behavior, and hence in respect to their functional
significance. The differences seen in the maturation-process have
thus far taught us nothing whatever in regard to the individual
physiological meaning of the chromosomes, in heredity or other-
wise, and they are not to be compared in value with the results of
direct experiment, such as those carried out by Boveri ('07) in
dispermic sea-urchin eggs. It is nevertheless of great interest that
the results from these different sources should be in harmony. In
my preceding paper I have called attention especially to the sig-
nificance of the couplings of the chromosomes, pointing out that
these certainly do not depend upon the size of the chromosomes
(though those which couple in synapsis are in fact equal members
of a pair save in certain special cases) nor can they, apparently,
depend upon the achromatic mechanism. The various combina-
tions in Metapodius seem to arise simply by the addition or sub-
traction of certain chromosomes without alteration of the achro-
matic elements; yet in the resulting new combinations the chro-
mosomes still behave each according to its kind, and (as previously
indicated) irrespective of their size. We seem thus driven to
accept the view that the chromosomes are physico-chemically dif-
ferent, with all the consequences which such a view may involve.
The cogency of the evidence in favor of the qualitative differ-
ences of the chromosomes brought forward in Boveri's masterly
work must be generally recognized, as has recently been admitted
even by Driesch ('09) who formerly endeavored to find a different
74 EDMUND B. WILSON
explanation of the facts. It will be evident to readers of my for-
mer papers that I am fully prepared to accept Boveri's conclusions;
but there is one very important fact, finally established by the
present paper, that must be clearly recognized. If we assume that
different factors of heredity are in some sense unequally distri-
buted among the chromosomes, we need feel no surprise that the
duplication of one or more of the ordinary chromosomes should
produce no perceptible qualitative effect upon development. But
it is very surprising that no visible effect should be produced by the
removal of a particular chromosome that has no duplicate to take
its place. In preceding papers I have called attention to the sing-
ular fact that Montgomery's material of M. terminalis differs con-
sistently from my own in the lack of the small idiochromosome or
u Y-element" (see Wilson '09a, for this term); but the possibility
of two distinct species or races having been confused could not be
absolutely excluded. In the present case, however, no doubt can
exist, since the two original specimens of M. femoratus from Miami,
Florida, are in my cabinet, and in perfect condition for identifica-
tion. One of these, as already stated, contains both the small
idiochromosome and an additional supernumerary, while both are
lacking in the other individual; yet the two individuals seem to be
otherwise in every essential respect identical. All doubt is thus
removed that the small idiochromosome or Y-element, which forms
the synaptic mate of the accessory chromosome or X-element,
may be present in some individuals and absent in others of the
same species, without the appearance of any corresponding dif-
ferences in the sexual or other characters as far as shown in the
external morphology of the animal. 9 This chromosome, as shown
9 It will be seen from this how readily discrepancies regarding the number of
chromosomes might arise between different observers working on the same species.
It might seem that we have here a simple and plausible explanation of the contra-
dictions that have arisen in the case of Anasa tristis; for we might assume that the
diploid number is 21 in some individuals of this species and in others 22 ; and a simi-
lar explanation has in fact already been adopted by more than one recent writer
(cf. Delia Valle: '09, Buchner: '096).
I am not myself able to take this view of this particular case for several reasons.
In the first place, if there be individuals of this species that have 22 spermatogonial
chromosomes, as maintained by Foot and Strobell ('07) we should expect to see
STUDIES ON CHROMOSOMES 75
by the work of Stevens and myself, is widely distributed among the
insects (Hemiptera, Coleoptera, Diptera) and is strictly confined
to the male line (except when supernumeraries are present). In
species having an odd or accessory chromosome the Y-element
(small idiochromosome) is wanting, and I have urged this fact as
showing that this latter chromosome cannot play any essential role
in sex-production 10 or in the transmission of the secondary sexual
characters, as Castle ('09) ingeniously suggests. What I desire
here to point out is that by parity of reasoning we should also con-
clude that this chromosome is devoid of any special significance in
heredity of any kind, at least as far as the visible external charac-
12 instead of 11 separate chromosomes in the first spermatocyte division, as we do
in the 22-chromosome individuals of Metapodius (Wilson: '09c). Throughout the
Hemiptera, indeed, when the accessory chromosome (or its homologue, the large
idiochromosome) is accompanied by a synaptic mate or Y-element, the two are
separate in the first division, which accordingly shows one more than the reduced
or haploid number i.e., *+! The photographs of Foot and Strobell show, how-
ever, 11 chromosomes in this division (the two m-chromosomes being of course
counted as one, like the other bivalents), as they should if the spermatogonial num-
ber be 21.
Still, this might be a case like that of Syromastes, where no Y-element is present,
but the accessory is itself double though such a parallel would hardly help the
case, since in no form is the failure of the accessory to divide in one division more
indubitably shown than in Syromastes, while Foot and Strobell are persuaded that
it does divide in Anasa. But, secondly, my own extended additional observation,
like the studies of Lefevre and McGill ('08), still continues to give but one result
as before. The living animals (from the same locality as the material of Foot and
Strobell) have been kept by hundreds in the greenhouse for months at a time in
successive years, and have been regularly employed for class work in cytology and
for experimental purposes, in the course of which large numbers of additional
sections and smears have been prepared and examined. Others as well as myself
have carefully searched among these preparations for cases showing more than 21
spermatogonical chromosomes, without success apart from the double or multiple
groups that occasionally appear. The same relation continually recurs, namely
21 spermatogonial chromosomes of which three are larger than the others, while
in the dividing ovarian cells the number 22 appears with equal constancy. That
not even one case of 22 spermatogonial chromosomes has thus far been found is
indeed surprising; for plus variations in the diploid groups are known to occur in
some species of Hemiptera, and I have myself described such cases (e. g., Wilson:
'09c).
These and other reasons lead me to believe that the conclusions of Foot and Stro-
bell were based on the observation either of very rare fluctuations in the normal
diploid number or of accidental products of the technique.
" Loc. cit.: '06, '09a, '09d.
76 EDMUND B. WILSON
ters and the more obvious internal features are concerned. Taken
by itself this case may seem to form a legitimate piece of evidence
against Boveri's theory. It cannot, howevei , be taken alone, but
must be viewed from the more general standpoint given by the
evidence as a whole. We are still too ignorant of the significance
of the " sex-chromosomes " to form an adequate opinion as to the
meaning of the Y-element. It may be in this case (as I earlier
suggested) a degenerating element, or may represent an excess of
a chromatin that is duplicated elsewhere in the chromosome-
group; but these and other speculative possibilites that suggest
themselves may well await the outcome of further study.
Department of Zoology, Columbia University, December 23, 1909.
BIBLIOGRAPHY
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STUDIES ON CHROMOSOMES 77
FOOT AND STROBELL. 1907 A Study of Chromosomes in the Spermatogenesis of
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1909 The Nucleoli in the Spermatocytes and Germinal Vesicles of
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1908 Spermatogenesis of Xiphidium fasciatum. Sci. Bull., Kansas
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genesis of Periplaneta Americana. Q. J. M. S., 48, 4.
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78 EDMUND B. WILSON
ROBERTSON, W. R. B. 1908 The Chromosome Complex of Syrbula admirabilis.
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1906 A Comparative Study of the Heterochromosomes in Certain
Species of Coleoptera, Hemiptera and Lepidoptera, with Especial
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1908 A Study of the Germ-cells of Certain Diptera, with Reference
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1909 Further Studies on the Chromosomes of Coleoptera. Ibid., 6, 1.
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1909 Histologische Beitrage, 7, Zeiptunkt der Bestimmung des
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1905c The Paired Microchromosomes, Idiochromosomes and Hetero-
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1909a Recent Researches on the Determination and Heredity of
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19096 The Accessory Chromosome in Syromastes and Pyrrhocoris.
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STUDIES ON CHROMOSOMES
VII. A Review of the Chromosomes of Nezara;
with Some More General Considerations
By EDMUND B. WILSON
Zoological Department, Columbia University
Reprinted from THE JOURNAL OF MORPHOLOGY
Vol. 22, No. 1, March 20, 1911
Reprinted from JOURNAL OF MORPHOLOGY, VOL. 22, No. 1
MARCH, 1911
STUDIES ON CHROMOSOMES
VII. A REVIEW OF THE CHROMOSOMES OP NEZARA; WITH SOME
MORE GENERAL CONSIDERATIONS
EDMUND B. WILSON
From the Zoological Department, Columbia University
NINE FIGURES AND ONE PLATE
CONTENTS
Introduction 1 71
Descriptive 73
1 The second spermatocyte-division in Nezara 73
a The idiochromosomes 73
b The double chromosome 77
2 The first spermatocyte-division 78
3 The growth-period and spermatocyte-prophases 80
4 The diploid chromosome-groups 83
General 84
5 The idiochromosomes 84
a Composition and origin of the XY-pair 85
b Modifications of the X-element 88
c Sex-limited heredity 94
d Secondary sexual characters 99
6 Modes in which the chromosome-number may change 99
Conclusion 105
INTRODUCTION
In the first of these 'Studies' ('05a) I described the idiochromo-
somes (X and Y-chromosomes) of Nezara hilaris as being of equal
size in the male, and reached the conclusion that in this species
no visible dimorphism appears in the spermatid-nuclei. In my
third 'Study' ('06), after examination of the female diploid groups,
this species was assigned a unique position as the single then
71
72 EDMUND B. WILSON
known representative of a type in which a pair of idiochromo-
somes can be identified in both sexes, but are of equal size in
both, and in which, accordingly, no visible sexual differences ap-
pear in the diploid nuclei. These conclusions, as is now appar-
ent, were based upon a wrong identification of the idiochromo-
some-pair, which is not the smallest pair, as I then believed, 1 but
one of the largest. When this fact was recognized, the true con-
ditions soon became evident.
I was led to re-examine Nezara hilaris by the fact (very sur-
prising to me) that in Nezara viridula, a southern species closely
similar to N. hilaris, the idiochromosomes of the male are ex-
tremely unequal in size, and the dimorphism of the spermatid-
nuclei is correspondingly marked. Upon returning to the study
of N. hilaris it soon became manifest that the dimorphism is
present in this species also, though in far less conspicuous form.
The size-difference between the X- and Y-chromosomes is here
often so slight that I did not at first distinguish it from an incon-
stant fluctuation of size, such as is sometimes seen between the
members of the other chromosome-pairs. When, however, the
identity of the XY-pair was correctly recognized, its constancy
of position and of size in the second division enabled me to make
an accurate comparison between it and the other bivalents; and
this fully established the constant inequality of its members,
which is constantly greater than that now and then seen in other
pairs. Both species also exhibit some other very interesting
features that I overlooked in my former studies.
Nezara can therefore no longer stand as a representative of
the third of the types distinguished in my third 'Study,' but
belongs with Euschistus, Lygaeus, etc., in the second type
1 This was in part because in most of the other forms known at the time the
idiochromosomes are in fact the smallest, or one of the smallest, pairs. In part,
also, I followed Montgomery ('01) who described in this species two small " chro-
matin nucleoli" in the spermatogonial groups, and believed them to be identical
with the chromatic nucleolus of the growth-period. In a later paper ('06) Mont-
gomery states these "chromatin nucleoli" to be "apparently not quite equal in
volume," and asserts that I was in error in describing them as equal. In my
material they are certainly equal in the great majority of cases. However, this
is not the idiochromosome-pair.
STUDIES ON CHROMOSOMES 73
DESCRIPTIVE
Since the two species agree very closely save in respect to the
idiochromosomes they may conveniently be considered together.
Before describing the divisions, attention may be called to a
striking difference between the two species in respect to the size
of the cells and karyokinetic figures. As a comparison of the
figures will show, the spermatocytes and maturation division-
figures of N. hilaris are much larger than those of N. viridula.
In the spermatogonia this difference is also apparent, though less
marked. In the ovaries, strange to say, it cannot certainly be
detected, either in the dividing cells or in the nuclei of the follicle-
cells or of the tip-cells at the upper end of the ovary. It would be
interesting to make a more accurate study of these relations;
but I will here only state that the differences between the two
species seem to arise mainly through greater growth of the
spermatocytes in N. hilaris. With this is correlated a greater
size of the testis as a whole; but the size of the entire body in this
species is but little larger, as far as I have observed, than in N.
viridula.
As regards the general features of the divisions, the diploid
groups of both sexes uniformly contain fourteen chromosomes,
the first spermatocyte-division eight and the second seven, the
idiochromosomes being, as is the rule in Hemiptera, separate and
univalent in the first division.
1. The second spermatocyte-division
a. The idiochromosomes. Polar views of the second division
always show 7 chromosomes which are usually grouped in an
irregular ring of six with the seventh near its center (fig. 3 j-m,
figs. 14, 15). In both species one chromosome of the outer ring
(s) can usually be distinguished as the smallest, though this is
not always evident owing to the apparent variations produced
by different degrees of elongation. This is the chromosome that
I formerly supposed to be the idiochromosome-bivalent, despite
its peripheral position, and despite the fact, which I had myself
described, that a similar small chromosome, also peripheral in posi-
JOUHNAL OF MORPHOLOGT, VOL. 22, NO. I
74
EDMUND B. WILSON
EXPLANATION OF TEXT FIGURES
Figures 1 to 9 are from camera drawings, and are not schematized except
that in a few instances the chromosomes have been artificially spread out in a
series in order to facilitate comparison. Figs. 2 k-l are somewhat more enlarged
than the others. In all the figures d denotes the double chromosome or 'd-chro-
mosome,' s the small chromosome, X the large idiochromosome and Y the small.
TK
*?*
%'.*.
^ /
Fig. 1 The second spermatocyte-division in Nezara viridula. a-d, metaphases
in side view; e-g, anaphases; h, i, polar views of two sister-groups, middle ana-
phase, from the same spindle and in the same section.
tion, appears in several other pentatomids (e.g., in Euschistus, Coe-
nus and Mineus). But Nezara forms no exception to the rule that
the central chromosome is the idiochromosome-bivalent. In N.
viridula this is immediately apparent in side views (often also in
polar views) where the central chromosome is seen to consist of two
very unequal components, the smaller being not more than one
fourth or one fifth the size of the larger (fig. 1 a-c). In the ana-
phases these separate and pass to opposite poles, while all the others
divide equally (fig. 1 e-g). Polar views of middle or rather late
anaphases, when both daughter-groups can be seen superposed
in the same section, clearly show the marked difference of the
two groups in respect to the idiochromosomes (fig. 1 h-i). All the
facts are here so nearly similar to those seen in Euschistus or
Lygaeus as to require no further description.
STUDIES ON CHROMOSOMES 75
In N. hilaris the conditions differ only in that the two compo-
nents of the central chromosome are but slightly unequal; but in
the examination of at least two hundred of these divisions I have
never failed to detect the inequality. A series of side views are
shown in fig. 2 a-i, figs. 16-21, two of which show all the chromo-
somes. These figures illustrate practically all the variations
that have been seen in the idiochromosomes. The most charac-
teristic condition is that seen in 2 a, b, d, in which both idiochro-
mosomes (X and Y) are more or less elongated and united end to
end. Less often one of them assumes a more spheroidal form
(fig. 2 e, h, i, fig. 17). The size-difference, though always evident,
seems to vary slightly (perhaps because one or the other compo-
nent may be more or less compressed laterally), but is always dis-
tinctly greater than that now and then seen in other bivalents.
Fig. 2 j shows a mid-anaphase 2 (cf. figs. 21-23) in which the
inequality would hardly be noticed without close study and the
comparison of other cases. Figs. 2 k and I are similar stages
showing all the chromosomes spread out in a series for the sake
of comparison. In both, the two idiochromosomes are easily
distinguishable, 3 and the larger is seen to be one of the three largest
chromosomes. Figs. 2 w-n, o-p, q-r and s-t are pairs of sister-
groups, in each case from the same spindle in anaphase. All of
these are selected from cases in which a distinct size-difference
appears between X and Y, but there are also many cases in which
this cannot be seen. Such a case was figured in fig. 4 e-f of my
first ' Study' the correctness of which is confirmed by re-examina-
tion of the original section. This condition is due simply to the
fact that the large idiochromosome is more elongated than the
small, so that the size-difference cannot be seen in polar view;
and for the same reason it is often not evident in polar views of
the metaphase.
2 This and the two following figures are a little more enlarged than the others .
3 Fig. 2 1 is the same group figured in fig. 4 d of my first 'Study,' carefully redrawn
and corrected. A comparison of the two drawings will show that in the latter a
distinct size-difference between X and Y is actually shown but is minimized by
the fact that the former is represented a trifle too small, the latter a little too
large. It is now also evident that they are connected by two connecting fibres
instead of by one.
76
EDMUND B. WILSON
Y * m **
-
o
r
Fig. 2 The second spermatocyte-division in Nezara hilaris. a-i, metaphase
figures in side view, o and e showing all the chromosomes; j-l, mid-anaphases; in
k and I all the chromosomes are shown artificially spread out in series; m-n, o-p,
q-r, s-t, four pairs of sister-groups from late anaphases, in polar view, in each case
from the same spindle.
STUDIES ON CHROMOSOMES 77
b. The double chromosome. A second interesting feature of the
second division that I formerly overlooked is the presence of a
remarkable double chromosome which in the metaphase has ex-
actly the appearance of a butterfly with widespread wings. This
chromosome (which may be called the d-chromosome) is shown in
profile view in 2 b-e and 1 a-d, 16, 17, 20, 24, 25. This is the only
chromosome in the second division that shows any approach to a
quadripartite form, and its characters are so marked as to constitute
the most striking single feature of the division. As the figures
show, it is one of the largest of all the chromosomes. It always
has an asymmetrical tetrad shape, giving exactly the appearance
of a smaller and a larger dyad in close union; and it always lies
in the outer ring, so placed as to undergo an equal division, and
with the larger wings of the butterfly turned towards the axis of
the spindle. In polar view (3 j-rri) the duality is far less apparent
and sometimes invisible, even upon careful focussing. In N.
viridula the duality is always apparent in side view, but the but-
terfly shape is usually less evident than in N. hilaris.
In the initial anaphases the d-chromosome divides symmetri-
cally, drawing apart into two bipartite chromosomes (2 j, k, 1 g) ;
but this is seldom evident save in profile view. Viewed from the
pole the duality does not now ordinarily appear, though it may
still sometimes be seen upon careful focussing. In the later ana-
phases the two components tend to fuse, and often can no longer
be distinguished. Not seldom, however, the duality is visible
even in the final anaphases; and sometimes this is so conspicuous
that the spermatid-group seems at first sight to comprise eight
instead of seven separate chromosomes (n, r, s, t).
Since the duality of this chromosome does not certainly appear
in the spermatogonial groups or in the first spermatocyte-division,
its peculiar form in the second division might be supposed to
result from some special mechanical relation to the spindle-fibers
in that division. This is, however, excluded by examination of
the interkinesis, in which the chromosomes are irregularly scat-
tered. In these stages, even when the spindle is still very small
and the chromosomes lie in a quite irregular group, the butterfly
shape is already perfectly evident; and it shows no constancy of
78 EDMUND B. WILSON
relation to the spindle-axis, often lying at right angles to the
latter. Apparently therefore its duality arises quite independ-
ently of the spindle or astral rays, and its constant position in
the fully formed spindle is the result of a later adjustment. In
this species, as in many others, each chromosome is connected with
the pole by a bundle of delicate fibers. In case of the d-chromo-
some this bundle is very broad, but I cannot be sure that it is
double.
At first sight any observer would, I think, take the d-chromo-
some to be merely a result of the accidental superposition or close
adhesion of two separate dyads of unequal size; but such an inter-
pretation is inadmissible. When all the chromosomes can be
unmistakably seen, the d-chromosome is found to constitute one
of the seven separate elements invariably present in this division;
and since the diploid number is 14 in both sexes this chromosome
must represent one chromosome, not two, of the original sperma-
togonial groups. It is certain, therefore, that the double appear-
ance does not result from close apposition of two separate chromo-
somes; it is therefore not a " tetrad" in the ordinary sense of the
word i.e., not one that results from the synapsis of two chromo-
somes that are originally separate in the diploid groups.
2. The first spermatocyte-division
This division requires only brief mention. As stated, it shows
eight separate chromosomes, of which the only one that can be
positively identified is the Y-chromosome of N. viridula. This
chromosome, always immediately recognizable in this species
by its small size (3 c, d, f, g, i), figs. 12, 13), is usually central in
position, like the m-chromosome of the Coreidae, but this is not
invariable. Since it divides equally, and without association with
any other chromosome (3 g) it is evident that the two idiochro-
mosomes must be separate and univalent in this division. In N.
hilaris (3 a, b, figs. 10, 11) the eight chromosomes usually form an
irregular ring, there is no central chromosome, and neither idio-
chromosome can be certainly recognized. It nevertheless seems
a safe inference from what is seen in N. viridula that the two
idiochromosomes are here also separate and univalent.
STUDIES ON CHROMOSOMES
79
a
c
XY-
XY
J
k
Fig. 3 First and second spermatocyte-divisions in the two species of Nezara.
a, 6, first division, hilaris, polar views: c, d, corresponding views of viridula; first
division, hilaris, side view showing five of the chromosomes in position and the
other three to the right above;/, corresponding view of viridula; g, middle ana-
phase, viridula, showing division of Y; h, first division metaphase, hilaris, all the
chromosomes artificially spread out in series; i, corresponding view of viridula;
j, k, second division metaphase, hilaris, polar views; I, m, corresponding views
of viridula.
80 EDMUND B. WILSON
In this division the d-chromosome can not be identified in either
species. Figs. 3 e, /, h, i, show all the chromosomes of the two
species, in each case from a single spindle in side view. Most
of them have a simple bipartite form, but in each species two or
three of them often appear more or less distinctly quadripartite
as is, of course, often the case with the bivalents in this division.
In N. hilaris one of the largest chromosomes is usually more
elongated than the others, and each half shows a slight trans-
verse constriction. I suspect that this may be the d-chromosome,
but cannot establish the identification.
3. The growth-period and spermatocyte-prophases
These stages fully bear out the conclusions based upon the
divisions and establish the identity of the idiochromosome-pair
with the chromatic nucleolus of the growth-period. Throughout
the growth-period each nucleus contains a single intensely stain-
ing spheroidal chromatic nucleolus and in addition a very large,
clearly defined pale plasmosome, which is sometimes double.
Series of drawings of these two bodies (in each case from the same
nucleus, and in their relative position) are given in figs. 4 i-l and
m-p, from cells of the middle growth-period. They are also
shown in figs. 26-29. In these stages no sign of duality is to be
seen in the chromatic nucleolus, even after long extraction or in
saffranin preparations. In later stages, as the chromosomes begin
to condense, this nucleolus becomes less regular in outline, and
gradually assumes a tetrad form, which becomes very clear as
the chromosomes assume their final shape. This transformation
may be traced without a break, successive stages being often seen
within the same cyst. Just before the nuclear wall breaks down
this tetrad is still clearly distinguishable from the others by its
asymmetrical quadripartite form, as seen in 4 y, z, which show all
the chromosomes (in each case from two successive sections).
Figs. 4 q-t show four views of this tetrad at this period in N.
hilaris, while u-x are corresponding views of N. viridula. These
figures (which might be indefinitely multiplied) show the marked
differences between the two species in respect to this tetrad,
obviously corresponding to that seen between the idiochromosome
* 9
V W
Fig. 4 The diploid groups, nucleoli of the growth-period, and late prophase-
figures ofthe two species of Nezara. a, b, spermatogonial groups, hilaris; c, d, the
same, viridula; e, f, ovarian groups, hilaris; g, h, the same, viridula; i-l, chromatic
nucleolus and plasmosome from the growth-period, in each case from the same
nucleolus in their relative position; m-p, corresponding views, viridula; q-t, the
idiochromosome-tetrad (chromatic nucleolus) from prophase nucleoli, hilaris;
u-x, corresponding views, viridula; y, late prophase nucleus, showing all the
chromosomes, hilaris (combination figure from two sections) ; z, corresponding
stage, viridula, three of the chromosomes from adjoining section at the right.
82 EDMUND B. WILSON
bivalents of the two in the second division. 4 The two species
may in fact readily be distinguished by mere inspection of the
chromatic nucleolus at this period. Already at this time the two
components are here and there seen to be separating, but as a
rule they do not finally move apart until the nuclear wall has
dissolved. From this time forward they cannot be individually
identified with exception of the small idiochromosome of N.
viridula, which is obvious at every period.
As far as my material shows, the earlier stages of the idio-
chromosomes can not be so readily traced in Nezara as in some
other species, and the chromatic nucleolus can not actually be fol-
lowed backward to the spermatogonial telophases as can be
done in such forms as Lygaeus or Oncopeltus, of which a detailed
account will be given in a later publication. The prophase -figures,
however, decisively establish its identity with an unequal pair of
chromosomes that divide separately in the first spermatocyte-
division; and in N. viridula, one of these is certainly the small
idiochromosome. It may therefore confidently be concluded
that the chromatic nucleolus is identical with the idiochromo-
some-pair, as in so many other cases. Comparison of the division-
figures proves that this pair can not be identical with the small
pair that I formerly supposed to be the idiochromosome-pair;
and this small pair is moreover usually recognizable in the pro-
phase groups (s, in 5 y, z) in addition to the unequal pair.
The foregoing facts make it clear that in Nezara the idiochromo-
somes undergo a process of synapsis at the same time with the
other chromosome-pairs, and that their separation before the first
division is a secondary process, to be followed by a second conju-
gation after this division is completed. A similar process often
takes place in many other Hemiptera. There are, however, some
forms, like Oncopeltus, in which the idiochromosomes are always
separate, from the last spermatogonial division through all the suc-
ceeding stages up to the end of the first division. In this case,
which I shall describe more fully hereafter, there can be no doubt
that the conjugation which follows the first division is a primary
synapsis, to be immediately followed by a disjunction.
4 Cf. the earlier figures of the corresponding tetrad in Brochymena in my first
'Study,' fig. 7.
STUDIES ON CHROMOSOMES 83
4. The diploid chromosome-groups
In these groups the interest centers again in the identity of the
idiochromosomes and the d-chromosome. Of the 14 separate
chrosomomes present in the diploid nuclei of both sexes, none
shows any constant indication of duality (figs. 4 a-h). The d-
chromosome can not, therefore, be identified in these stages.
Secondly, in both species the diploid groups of the two sexes show
the same relation as in other Hemiptera of this type, though this
is, of course, more readily seen in N. viridula than in hilaris, owing
to the small size of the Y-chromosome. In the spermatogonial
groups of this species (4 c, d) this chromosome is at once recog-
nizable while in the female groups (g, ti) it is lacking, its place
being taken by one of larger size. In both sexes the small pair
(s, s) is also recognizable. In this species, accordingly, the Y-
chromosome is confined to the male line, and the Y-class of
spermatozoa must be male-producing, as in other forms.
In N. hilaris the Y-chromosome can not be identified (4 a, 6),
but the relation of the spermatozoa to sex-production is shown in
another way, though less unmistakably than in N. viridula. As
already described, the large idiochromosome or X-chromosome is
one of the largest three chromosomes seen in the second division.
We should therefore expect to see five largest chromosomes in
the male diploid groups. This is clearly apparent in figs. 4 a, 6,
and is also shown in the corresponding figures of N. viridula
(c, d) though not quite so clearly. One of these five in the male
should be the X-chromosome; and if the usual relation of the
spermatozoa to sex hold true, there should be six largest chro-
mosomes in the diploid groups of the female. This relation actu-
ally appears in nearly all cases, and is shown in figs. 4 e, f, g, h, in
each of which, again, the small pair (s, s) may be recognized.
Though this evidence is in itself less convincing than that afforded
by N. viridula (since the relation can not always be made out
with certainty) it is fully in harmony with the latter, and sustains
the same conclusion. 5
5 This relation is shown in my original figures of N. hilaris, though not quite
as clearly as in the groups here figured. In my first 'Study' ('05) the five largest
chromosomes are very clearly shown in fig. 4 h, and are also evident in 4 q. In the
third 'Study' the relation is hardly evident in the male but fairly so in the
female (figs. 5 I, TO).
84 EDMUND B. WILSON
GENERAL
5. The idiochromosomes
The case of Nezara shows how readily a morphological dimorph-
ism of the spermatid-nuclei may be overlooked when the X- and
Y-chromosomes do not differ markedly in size ; and it emphasizes
the necessity for the closest scrutiny of these chromosomes in the
study of this general question. In my fourth 'Study' I placed
with Nezara hilaris, as a second example of my original 'third
type/ the lygaeid species Oncopeltus fasciatus (Dall.), on the
strength of Montgomery's account of the conditions in the male
('01, '06) and my own unpublished observations on both sexes.
While I have carefully re-examined this case also, I am not yet
prepared to express an unqualified opinion in regard to it. Cer-
tainly, in very many of the cells of this species, at every period of
the spermatogenesis, the idiochromosomes (which are always sep-
arate up to the second division) seem to be perfectly equal. A
slight inequality may indeed be seen in some cases ; but as far as I
can yet determine this seems to fall within the range of the size-
variation in other chromosomes and gives no positive ground for
the recognition of a morphological dimorphism in the spermatozoa.
A similar condition has been described in several other insects, not-
ably in some of the Lepidoptera (Stevens, '06 ; Dederer, '08 ; Cook,
' 10) , in the earwig Anisolaba (Randolph, '08) and apparently also
in the beetle Hydrophilus according to Arnold ('08). I see no rea-
son to question these observations; but the interpretation to be
placed on them is by no means clear at present. The experimental
evidence on the Lepidoptera seems to demonstrate that in at least
one case that of Abraxas according to Doncaster and Raynor,
it is the eggs and not the spermatozoa that are sexually dimorphic ;
that is, in the terms that I have recently suggested ('10a), in
this case it is the female that is sexually 'digametic' while the
male is 'homogametic.' Baltzer's careful work on the sea-
urchins ('09) clearly demonstrates a cytological sexual dimorphism
in the mature eggs of these animals, and shows that the sperm-
nuclei are all alike. In cases, therefore, where no visible dimorph-
ism of the spermatid-nuclei is demonstrable, two possibilities
STUDIES ON CHROMOSOMES 85
are to be considered, namely, (1) that it may be the female which
(as in sea-urchins) is the digametic sex, and (2) that one sex or
the other may still be physiologically digametic even though this
condition is not visibly expressed in the chromosomes. The
first of these possibilities may readily be tested by cytological
examination of the female groups. The second can only be
examined by means of experiment, and especially by experiments
on sex-limited heredity. It is interesting that the work of Don-
caster and Raynor, cited above, and the more recent one of Morgan
on Drosophila ('10) have given exactly converse results, the former
demonstrating a sexual dimorphism of the eggs, the latter of the
spermatozoa. This agrees with the cytological data, as far as
they have been worked out. The researches of Stevens ('08, 10),
on the Diptera establish the cytological dimorphism of the sper-
matozoa in these animals, while all observers of the Lepidoptera
have thus far failed to find such dimorphism in this group. It
thus becomes a very interesting question whether a cytological
dimorphism of the mature eggs may be demonstrable in the
Lepidoptera; though a failure to find it would in no wise lessen
the force of the experimental data. Physiological differences be-
tween the chromosomes are of course not necessarily accompanied
by corresponding morphological ones indeed such a correlation
is probably exceptional.
(1) (a) Composition and origin of the XY-pair. The facts
seen in Nezara again force upon our attention the puzzle of the
Y-chromosome or 'small idiochromosome.' It is remarkable
that two species so nearly akin as N. hilaris and N. viridula should
differ so widely in respect to this chromosome; though this is
hardly so surprising as the fact that in Metapodius this chromo-
some, as I have shown ('09, '10) may actually either be present or
absent in different individuals of the same species. These facts
show, as I have urged, that although the Y-chromosome shows a
constant relation to sex when it is present, it can not be an essen-
tial factor in sex-production. As the case now stands this might
be taken as a direct piece of evidence against the view that the
idiochromosomes are concerned with sex-heredity. Further, as
I have pointed out ('10) in Metapodius the introduction of super-
86 EDMUND B. WILSON
numerary Y-chromosomes into the female has no visible effect
upon any of the characters of the animal, sexual or otherwise;
and this might be urged against the whole conception of qualita-
tive differences among the chromosomes and of their determina-
tive action in development. It is especially in view of these
and certain other general questions that I wish to indicate some
of the many possibilities that must be taken into account in the
consideration of this problem. My discussion is throughout
based upon the assumption that the chromosomes do in fact
play some definite role in determination, and that they exhibit
qualitative differences in this respect. I do not hold that they
are the exclusive factors of determination; though it is often con-
venient, for the sake of brevity, to speak of them as if they were
such.
(2) Cytologically considered, the morphological dimorphism
of the spermatozoa seems to have arisen by the transformation
of what was originally a single pair of chromosomes comparable
to the other synaptic pairs. We have at present no information
as to whether the members of this pair were equal or unequal in
size ; but in either case there are grounds for the assumption that
its two members differed in some definite way in respect to the
quality of the chromatin of which they were composed. This
pair, which may be called the primitive XY-pair, has undergone
many modifications in different species, but without altering its
essential relation to sex. In the insects (Hemiptera, Coleoptera,
Diptera) its most frequent condition is that of an unequal pair, con-
sisting of a 'large idiochromosome' or 'X-chromosome/ and a
" small idiochromosome" or ' Y-chromosome,' the latter being con-
fined to the male line, while the former appears in both sexes
single in the male and paired in the female. That all gradations
exist between cases where X and Y are very unequal (as in many
Coleoptera and Diptera and in some Hemiptera) and those in which
they are nearly or quite equal (Mineus, Nezara, Oncopeltus) gives
some ground for the conclusion that in the original type the
XY-pair was but slightly if at all unequal.
By disappearance of the free Y-member of this pair has arisen
the unpaired odd or 'accessory' chromosome, which accordingly
STUDIES ON CHROMOSOMES 87
has no synaptic mate. This condition seems to have arisen in
more than one way. It is almost certain that in many cases the
Y-chromosome has disappeared by a process of gradual and pro-
gressive reduction (as indicated by the graded series observed
in the Hemiptera (Wilson, '056, '06). In some cases (of which
Metapodius is an example) the same result may have been pro-
duced suddenly by a failure of the idiochromosomes to separate
in the second spermatocyte-division (Wilson, '096). A third pos-
sibility, first suggested by Stevens ('06), is that the X-element
may have separated from a YY-pair with which It was originally
united. This possibility seems to be supported by recent obser-
vations on Ascaris megalocephala, where the X-chromosome is
sometimes fused with one of the other pairs, sometimes free
(Edwards, '10).
(3) We have as yet no positive knowledge as to how the X-
member of the XY-pair originally differed, or now differs, from
the Y, or as to how this difference arose a definite answer to
these questions would probably give the solution of the essential
problem of sex. There are, however, pretty definite grounds for
the hypothesis that the X-member contains a specific 'X-chroma-
tin' that is not present in the Y-member, and that the XY-pair
is heterozygous in this respect. If this be so, the primary sexual
differentiation is therefore traceable to a condition of plus or
minus in this pair, accompanied by a corresponding difference
between the nuclear constitution of the two sexes. (Cf. Wilson,
'10a.) Further, there is also reason for regarding the heterozy-
gous condition of this pair as due to the presence of the X-chroma-
tin in one member of a pair which is (or originally was) homozy-
gous in respect to its other constituents. The latter may be
called collectively the 'Y-chromatin'; and we may, accordingly,
think of the XY-pair as being essentially a YY-pair with one
member of which the X-chromatin is associated. 6 Both the X-
6 This suggestion is in principle the same as one earlier made by Stevens ('06,
p. 54) that the Y-chromosome represents "some character or "characters which
are correlated with the sex-character in some species but not in others," with one
member of which the X-chromosome is fused; and that "a pair of small chromo-
somes might be subtracted from the unequal pair, leaving an odd chromosome."
88 EDMUND B. WILSON
chromatin and the Y may themselves be composite, thus giving
the possibility of many secondary modifications. The point of
view thus afforded opens many possibilities for an understanding
of sex-limited heredity, as indicated beyond.
(6) Modifications of the X-element. This view of the XY-pair
is based upon two series of facts, of which the first includes the
various modifications of the X-member of the pair seen in dif-
ferent species. It is, perhaps, most directly suggested by a study
of the pentatomid species Thyanta custator. In this common and
widely distributed species I have found two races, which thus far
can not be distinguished by competent systematists, 7 but which
differ in a remarkable way in respect to both the total number
of chromosomes and the XY-pair. In one of these races (which
I will call the 'A form'), widely distributed throughout the south
and west, the total number in both sexes is 16, and the XY-pair
of the male is a typical unequal pair of idiochromosomes, exactly
like that seen in many other pentatomids (e.g., Euschistus,
Coenus or Banasa) . These are shown in fig. 5 a, b, their mode
of distribution being the usual one. The second race (the 'B
form') is thus far known from only a single locality in northern
New Jersey. It differs so remarkably from the A form that I
could not believe the observations to be trustworthy until repeated
study of material, collected in four successive years, established the
perfect constancy of the cytological conditions and the apparent
external identity of the two forms. In this race the XY-pair is
represented by three small chromosomes of equal size, which are
always separate in the diploid groups and in the first spermato-
cyte-di vision (fig. 5i), but in the second division are united to
form a linear triad series (5 c, d) . This group so divides that one
component passes to one pole and two to the other (5 e, Ji) , the
7 I am indebted to Mr. E. P. Van Duzee for a careful study of my whole series
of specimens of both races. He could find no constant differential between them.
Additional studies of this material are now being made by Mr. H. G. Barber.
Addendum. Since this paper was sent to press Mr. Barber, after prolonged
study, has reported his conclusion that the 'A form' is Thyanta custator of
Fabricius, while the 'B form' is probably Thyanta calceata of Say, which has
long been regarded as a synonym of former species.
STUDIES ON CHROMOSOMES
89
J
Y
ft
t
X
to
m
?* 'I 1 ' 1 |
^ P
r
i' 4
r
i
A:
"5.
r
Y
\
X
I
*
/'
ft
X
Fig. 5 Comparison of the XY-group in various Hemiptera. (a-i are orig-
inal; the others from Payne.) a, 6, Thyanta custator, 'A form,' second division
in side view; c, d, corresponding views of the 'B form'; e-h, anaphases of same;
i, polar view of first division of same; j, k, metaphase chromosomes, second divi-
sion, Diplocodus exsanguis; i, similar view of Rocconota annulicornis; TO, similar
view of Conorhinus sanguisugus; n, Sinea diadema; o, Prionidus cristatus; p,
Gelastocoris oculatus; q, anaphase chromosomes of the same species; r, the XY-
group, from the second division, Achollamultispinosa; s, diagram, slightly modified
from Payne, to show the distribution of the XY-components in the second divi-
sion of the same species.
JOURNAL OF MORPHOLOGY, VOL. 22, NO. 1
90 EDMUND B. WILSON
latter being usually in close contact and in later anaphases some-
times hardly separable (50), though now and then all three compo-
nents are for a time strung separately along the spindle in the
early anaphases, so that no doubt of their distinctness can exist
(5 /) . Comparison of the diploid groups of the two sexes shows
that those of the male contain but three of these small chromo-
somes and those of the female four, the total respective num-
bers being 27 and 28 (instead of 16 in both sexes, as in the A
form).
These facts make it perfectly clear that one of the small chromo-
somes in the male passes to the male-producing pole, and therefore
corresponds to the Y-chromosome; while the other two, taken to-
gether, represent the large idiochromosome, or X-chromosome, of
the A form precisely as in the reduvioids the single X-chromosome
of Diplocodus is represented by a double element in Fitchia,
Rocconota or Conorhinus (Payne). Had we no other evidence
on this point we might assume simply that the original X-chro-
mosome has divided into two equivalent X-chromosomes. But
there are other facts that give reason for the conclusion that the
breaking up of a single X-chromosome into separate components
means something more than this. In the B form, as in Fitchia or
Rocconota (fig. 5 I), the X-element consists of two equal compo-
nents, but in Conorhinus the two components are always of un-
equal size (5 ra). In Prionidus and in Sinea there are three equal
components (5 n, o), in Gelastocoris four equal ones (5 p, and
in Acholla multispinosa five, of which two are relatively large
and equal and three very small (5 r, s). In every case these com-
ponents, though quite separate in the diploid groups (and usually
also in the first spermatocyte-division) act as a unit in the second
division, though not fused, and pass together to the female-produc-
ing pole (Payne, '09, '10).
In the foregoing examples the X-element is accompanied by a
synaptic mate or Y-chromosome. The following are examples of
a similar breaking up of the X-element into separate components
when such a synaptic mate is missing. In Phylloxera (Morgan)
the X-element consists of two unequal components, sometimes
separate, sometimes fused together. In Syromastes (Gross,
STUDIES ON CHROMOSOMES 91
Wilson) it consists of two unequal components, always separate,
in the diploid groups but closely in contact (not fused) in both
spermatocyte-divisions. The recent work of Guyer ('10) indi-
cates a similar condition in the X-element of man. In Agalena
(Wallace) there are two equal components, always separate.
Finally, in Ascaris lumbricoides (Edwards, '10) there are five com-
ponents, separate, and scattered in the diploid groups but closely
associated in the spermatocyte-divisions.
In all these cases the significant fact is that not only the number
but also the size-relations of these components are constant; and
in many of these forms this fact may be seen in such multitudes
of cells, and with such schematic clearness, as to leave no manner
of doubt. It seems impossible to understand this series of phe-
nomena unless we assume that the single X-chromosome is essen-
tially a compound body i.e., one that consists of different con-
stituents that tend to segregate out into separate chromosomes.
We are led to suspect, further, that the composition of the X-
element, even when it is a single chromosome, may differ widely
in different species because of its great variations of size as between
different species. For instance, in the family of Coreidae it is
in some cases very large (Protenor), in others of middle size (Che-
linidea, Narnia, Anasa), in others one of the smallest of the chromo-
somes (Alydus). Similar examples might be given from other
groups.
In the case of Thyanta, therefore, it seems a fair assumption
that the double X-element of the B form likewise represents at
least a partial segregation of the X-chromatin from other con-
stituents; and the latter may plausibly be regarded as represent-
ing the 'Y-chromatin' of the original X-member of the pair.
In other words, we may think of the triad element as a YY-pair,
one member of which is accompanied by a separate X-chromosome.
In accordance with this its formula should be X.Y.Y, while that
of the A form is XY.Y; and this may also be extended to other
forms of similar type. If this be admissible, the male formula, as
regards essential chromatin-content, becomes in general XY.Y
and the female XY.XY, both sexes being homozygous for the
Y-constituents, while in respect to X the male is heterozygous,
92
EDMUND B. WILSON
the female homozygous. The puzzle of the Y-chromosome
would thus be solved; for although a separate Y-chromosome,
when present, is confined to the male line, its disappearance
only reduces the male from a homozygote to a heterozygote in
respect to the Y-chromatin, and the introduction of supernumer-
ary Y-chromosomes into the female (as in Metapodius) brings in
no new element.
L
a
A
I
Fig. 6 Compound groups formed by union of the X-chromosome with other
chromosomes in the Orthoptera. (a and b, from Sin6ty, the others from McClung. )
a, triad group, first division of Leptynia, metaphase; b, division of similar triad in
Dixippus; c, triad group formed by union of the X-chromosome with one of the
bivalents, first spermatocyte-prophase, Hesperotettix; d, the same element from
a metaphase group; e, the same element in the ensuing interkinesis ; /, the com-
pound element of Mermiria, from a first spermatocyte prophase; g, the same ele-
ment in the metaphase (now, according to McClung, united to a second bivalent
to form a pentad) ; h, the same element after its division, in the ensuing telophase.
The same general view as^hat outlined above is suggested by the
constant relation known to exist in some cases between the X-
chromosome and a particular pan- of the 'ordinary chromosomes/
The first observed case of this was recorded by Sinety ('01) in
the phasmid genera Leptynia and Dixippus (fig. 6 a, b), where the
X-chromosome is always attached to one of the bivalents in the
STUDIES ON CHROMOSOMES 93
first spermatocyte-division, and passes with one half of the bivalent
to one pole. Since the spermatogonial number in Leptynia (36)
is an even one and twice that of the separate chromosomes present
in the first spermatocyte-division, it may be inferred that the
X-element is already united with one of the ordinary chromo-
somes in the spermatogonia, though Sinety does not state this.
Somewhat later McClung ('05) discovered essentially similar rela-
tions in the grasshoppers Hesperotettix and Anabrus (fig. 6,
c-e),and in case of the first named form was able to establish the
important fact that it is always the same particular bivalent with
which the X-chromosome is thus associated. In respect to the
intimacy of this association, a progressive series seems to exist,
since in Leptynia it seems to take place in the spermatogonia, in
Hesperotettix only in the prophases of the first spermatocyte-
division, while in Thyanta the union is only effected after the
first division is completed.
Finally, the recent observations of Boring ('09), Boveri ('09)
and Edwards ('10) seem to establish the fact that in Ascaris megalo-
cephala the X-element, whether in the diploid groups or in the
maturation-divisions, may either appear as a separate chromo-
some (which has the usual behavior of an accessory chromosome)
or may be indistinguishably fused with one of the ordinary chromo-
somes.
These relations may, of course, be the result of a secondary
coupling; and I myself formerly so interpreted them ('09c). But
in view of what is seen in Thyanta or the reduvioids we may
well keep in mind the possibility that they are expressions or
remnants of a more primitive association, like that which I have
assumed for an original XY-pair. Whatever be their origin, the
effect is the same a definite linking of the X-chromatin with
that of one of the other pairs.
Fig. 7 shows, in purely schematic form, the general conception
of these relations that has been suggested above, the X-chromatin
being everywhere represented in black. A is the primitive XY-
pair from which all the other types may have been derived. By
simple reduction of such a pair arises the ordinary or typical
idiochromosome-pair (B) ; and from either A or B may be derived
94
EDMUND B. WILSON
the other types (C-G), 8 or the more complicated ones shown in
fig. 5. I represents the possible mode of separation of the
X-element from a YY-pair, as suggested by Stevens; and this
may be realized in Ascaris megalocephala (H). J and K are
schemes of the relations seen in Hesperotettix, Anabrus and
Mermiria (cf. fig. 6). These may be direct derivatives of a
primitive XY-pair, as the diagram suggests, or may be a result
C. Thyanta
X
XY-Pair
I
Idiochromosome
Pair
Y
,1 I
T.? I
D. Fitchia
.. Conorhinus
X
G. Syrornastes
Mermiria,
/i.Ascaris
' Jfesjberofattix
Fig. 7 Diagram illustrating the possible relation of the various types of idio-
chromosomes to a primitive XY-pair. Explanation in text.
of secondary coupling of X with other elements. In either case
X may itself have such a composition as is indicated in F (Prote-
nor).
(c) Sex-limited heredity. (1) The foregoing considerations have
an important bearing on the problem of sex-limited heredity,
for they give us a very definite view of how such heredity may be
effected. It is not my intention to consider this subject in ex-
8 These figures are not intended to indicate the precise mode of segregation of
the X- and Y-chromatins of the X-element, but only illustrate possible modes.
STUDIES ON CHROMOSOMES 95
tenso; but I wish to indicate some of the possibilities that have
been opened by the cytological results, even at the risk of offering
what may be regarded as too speculative a treatment of the matter.
It is obvious that any recessive mutation should exhibit sex-limited
heredity when crossed with the normal or dominant form, if it be due
to a factor contained in (or omitted from) the X-element. For in-
stance, in the remarkable Drosophila mutants discovered by
Morgan ('10) the experimental data establish the fact that white
eye-color (which seems to follow the same type of heredity as
color-blindness in man) is linked with a sex-determining factor in
such a way that when the white-eyed male is crossed with the
normal red-eyed female, the former character is neyer transmitted
from father to son, but through the daughters to some of the
grandsons (theoretically to 50 per cent), though the daughters are
not themselves white-eyed; that is, after such an initial cross, white
eyes fail to appear in the Fi generation in either sex and in the
F 2 generation appear only in some of the males. As Morgan
points out, this follows as a matter of course if the factor for white
eye be identical with, or linked with, a sex-determining factor in
respect to which the male is heterozygous or simplex, the female
homozygous or duplex. The X-element exactly corresponds in
mode of distribution to such a sex-determining factor; for this
chromosome, too, is simplex in the male, duplex in the female
and its introduction into the egg by the spermatozoon produces
the female condition, its absence the male. This chromosome
therefore, as I have shown ('06), is never transmitted from father
to son, but always from father to daughter. Conversely, the
male zygote always receives this chromosome from the mother.
So precise is the correspondence of all this with the course of sex-
limited heredity of this type that it is difficult to resist the con-
clusion that we have before us the actual mechanism of such
heredity in other words, that some factor essential for sex is
associated in the X-element with one that is responsible for the
sex-limited character.
This will be made clearer by the accompanying diagram (fig.
8) where the X-element assumed to be responsible for a recessive
sex-limited character is underscored (X) . This character may
96
EDMUND B. WILSON
be regarded as due to the absence of some particular constituent
that is present in the normal X-element.
Fig. 8 Diagram of the distribution of the X- and Y-elements in successive
generations, illustrating sex-limited heredity. The underscored X-element (X)
is assumed to bear a factor for a recessive character (e.g., white eye-color), while
X represents the normal or dominant character (e.g., red eye-color). Y (being the
absence of X) likewise represents the recessive character.
Upon pairing the affected male (XY) with the normal female
(XX) there are in the F, generation but two possible combina-
tions, XX and XY. The affected X-chromosome here passes
STUDIES ON CHROMOSOMES 97
into the female, and the male is normal ; but the female of course
likewise shows only the normal (dominant) character. In the
following F2 generation (5) there are four possible combinations
XX, XX, XY and XY, two of each sex. Though X is present in
half of each sex, the character appears only in the males, XY,
again because of its recessive nature. By crossing together males
of the composition XY and females of composition XX, some of
the resulting females will have the composition XX, and the sex-
limited character is thus made to appear in the female.
When the female is the heterozygous or digametic sex as in
sea-urchins, in Abraxas, the Plymouth Rock fowls, etc. exactly
the converse assumption has to be" made. Here, as Spillman
('08) and Castle ('09) have pointed out, the observed results
follow if the sex-limited character (e.g., lacticolor color-pattern
in Abraxas) be allelomorphic to, or the synaptic mate of, a sex-
determining factor, X, that is present as a single element in the
fe'male but absent in the male. The formulas now become 9
(as Spillman has indicated) XG (9 grossulariata), GG (cT gross.)
XG (9 lacticolor) and GG (d 1 lact). XG X GG then gives
in FI XG and GG (gross. 9 and cf), G having passed from the
female to the male. The following cross, XG X GG gives in F 2
the four types XG, XG, GG and GG, i.e., grossulariata appear-
ing in both sexes but lacticolor only in the female. By crossing
XG with GG some of the progeny will have the composition GG
(d" lacticolor). The other combinations follow as a matter of
course.
This interpretation is in all respects the exact converse of
that made in the case of Drosophila, which is also the case with
9 These formulas are in substance the same as those stated by Mr. Spillman in
a private letter to the writer, and are a simplified form of those suggested by Castle
('09). The interpretation thus given seems both the simplest and the most satis-
factory from the cytological point of view of all those that have been offered. It
obviates the cytological difficulties that I urged ('09) against Castle's formulas,
and renders unnecessary the secondary couplings that I suggested. All these
ways of formulating the matter conform, of course, to the same principle and
differ only in details of statement. Whether the synaptic mate of X is directly
comparable to the Y-chromosome of other insects (in which case the female formula
becomes XY and the male YY) is an open question.
98 EDMUNP B. WILSON
the experimental results, as Morgan has pointed out. It seems
probable that all the observed phenomena may be reduced in
principle to one or the other of these schemes, though many modi-
fications or complexities of detail probably exist. A possible basis
for many such modifications seems to be provided by the cyto-
logical facts already known.
(2) We might assume that in cases of the first type (e.g., Droso-
phila) both sex and the characters associated with it are deter-
mined by the same chromatin; and such a possibility should
certainly be borne in mind. But in view of the widely different
nature of the characters already known to exhibit sex-limited hered-
ity it seems improbable that we can regard them as all alike due
to the same chromatin. In the light of the conclusions that have
been indicated in regard to the composition of the X-element, it
seems more probable that such characters may be determined by
the various other forms of chromatin (' Y-chromatin') associated
with the X-chromatin. If these constituents be identical with
those contained in the free Y-chromosome (the synaptic mate of
X) sex-limited heredity would of course not appear, since the two
members of the pair would be homozygous in this respect. It
should make its appearance as a result of the dropping out, or
other modification, of certain Y-constituents of the X-element, and
such a mutation might arise in either sex.
We may perceive here the possibility of understanding many
different kinds of sex-limited heredity, perhaps of complex types
that have not yet been made known. Such a possibility is sug-
gested, for example, by the remarkable relation discovered by
McClung ('05) in Mermiria (fig. Qf-h, fig. 7 in diagram), where
the X-chromosome is in the first spermatocyte-division attached
at one end to a linear chain of four other elements to form a
pentad complex, to which may be given the formula XA . ABB.
This so divides as to separate XA from ABB. The interpretation
to be placed upon this is a puzzling question under any view, and
apparently must await more extended studies on both sexes, per-
haps also on other forms, before it can be fully cleared up. Even
here the possibility exists, I think, that the entire complex may
have arisen by the differentiation of a single original XY-pair;
STUDIES ON CHROMOSOMES 99
but this question is clearly not yet ready for discussion. How-
ever such associations have arisen, the result is equally appli-
cable to the explanation of sex-limited heredity.
(d) Secondary sexual characters. Castle ('09) has offered the
interesting suggestion that the free Y-chromosome may be re-
sponsible for the determination of secondary sexual characters
in the male. Though I have criticized this view ('09c) I now
believe it may be true for certain cases. It is obviously excluded
when the Y-chromosome is missing; and since nearly related
species in Metapodius even different individuals of the same spe-
cies show the same or similar secondary male characters whether
this chromosome be present or absent, it seems probable that
these characters are in general determined in some other way.
But if, as I have suggested, sex-limited heredity may arise through
a modification of the Y-constituents of the X-element, it follows
that the YY-pair thereby becomes heterozygous. In such case,
the free Y-chromosome, being confined to the male line, should
continue to represent characters that are no longer present in
the female, and hence would be indistinguishable from secondary
male characters otherwise determined. It has further become
evident (as is indicated below) that the chromosome-groups are
so plastic that their specific composition may vary widely from
species to species. It may very well be, therefore, that Castle's
suggestion may apply to some forms.
6. Modes in which the chromosome-number may change
The constant and characteristic duality of the 'd-chromosome'
in the second division suggests a series of questions regarding the
mode in which the chromosome-number may change that have
an important bearing on those already considered. The appear-
ance of this chromosome must suggest to any observer that it is
a compound body, consisting of two closely united components
that are invariably associated in a definite way; but it is especially
noteworthy that its duality does not certainly appear before the
last division. This case must be added to the steadily increasing
evidence that chromosomes which appear single and homoge-
100 EDMUND B. WILSON
neous to the eye may nevertheless be compound bodies. An
important part of it is derived from the modifications of the X-
element reviewed above; but the evidence is now being extended
to the 'autosomes' or ordinary chromosomes as well. The double
chromosome of Nezara suggests either the initial stages of a sep-
aration of one chromosome into two or the reverse process in
either case that we have before us one way in which the number,
and the composition, of the chromosomes may change from species
to species. This is supported by the recent observations of
Miss E. N. Browne ('10) on Notonecta. In N. undulata there
are always, in addition to a typical unequal XY pair, two small
chromosomes that appear in all the divisions as separate elements.
In N. irrorata there is always but one such chromosome, the total
number in each division being accordingly one less than in N.
irrorata. N. insulata presents a condition exactly intermediate,
there being sometimes one and sometimes two such small chromo-
somes. When, however, but one seems to be present, the second
may often be seen closely adherent to one of the larger chromo-
somes; and the latter may positively be identified, by its size, as
always the same one. It can hardly be doubted, as the author
points out, that we here see three stages in a process by which
the chromosome-number is changing, either by the fusion of two
originally separate chromosomes, or by the separation of one into
two. It is of the utmost importance that this process affects
a chromosome that can be positively identified as the same in
each case; for this proves that the change is not an indefinite
fluctation but the expression of a perfectly orderly process.
While there is here (as in the case of the d-chromosome of Nezara)
no way of knowing in which direction the change is taking
place, either alternative involves the conception that the indivi-
dual chromosomes may be compound bodies, whether as a re-
sult of previous fusion or as possible starting points for a subse-
quent segregation.
The facts now known indicate at least four possible ways in
which the chromosome-number (and in three of these also the
composition of the individual chromosomes, may change from
species to species.
STUDIES ON CHROMOSOMES 101
One is that suggested by the foregoing phenomena, i.e., the
gradual fusion of separate chromosomes into one or the reverse
process.
A second mode may be the gradual reduction and final disap-
pearance of particular chromosome-pairs, as was suggested by
Paulmier ('99), and afterwards by Montgomery and myself, in
case of the microchromosomes, or 'm-chromosomes' of the co-
reid Hemiptera. In respect to the size of these chromosomes, a
graded series may be traced from forms in which they are very
large (as in Protenor) through those where they are of intermediate
size down to cases where they are very small (as in Archimerus)
and finally to such a condition as that seen in Pachylis (fig.
9 j-l) where they are almost as minute as centrioles and may
almost be regarded as vestigial. Four of these stages are shown
in fig. 9. In Protenor (a-c) the m-chromosomes are so nearly
of the same size as the next smallest pair that they often can not
be positively identified in the spermatogonial groups. In Lepto-
glossus phyllopus (d-f) they are always recognizable, though not
much smaller than the next pair. In L. oppositus or L. corcu-
lus they are a little smaller. In Anasa (the form in which they
were first discovered by Paulmier) they are of middle size (g-i) ,
representing perhaps a fair average of the group. Several other
genera (e.g., Metapodius) show intermediate stages between this
condition and that seen in Archimerus (figured in my second
'Study,' and more recently by Morrill) where the m-chromo-
somes are almost as small as in Pachylis. It is most remarkable
that throughout this whole series the m-chromosomes exhibit
essentially the same behavior (Wilson, '056, '06), usually remain-
ing separate throughout the entire growth-period and only con-
jugating in the final prophases of the first spermatocyte-division,
to form a bivalent which with rare exceptions occupies the center
of the metaphase group; in some forms, also (e.g., Protenor, Aly-
dus) they show a marked tendency to condense at a much earlier
period than the other chromosomes. The m-chromosomes of
Pachylis, excessively minute though they are, exhibit a behavior
in all respects as constant and characteristic as even the largest
of the series. In the Lygaeidae they seem to be present in some
102
EDMUND B. WILSON
X
k
Fig. 9 Comparison of the m-chromosomes in Hemiptera. (In each horizon-
tal row are shown at the left a spermatogonial group, in the middle a polar view
of the first spermatocyte-division, at the right a side-view of the same division.)
a-c, Protenor belfragei; d-f, Leptoglossus phyllopus; g-i, Anasa tristis; j-l,
Pachylis gigas.
STUDIES ON CHROMOSOMES 103
species (Oedancala, t. Montgomery), in others absent (Lygaeus).
In the Pyrrhocoridae (Pyrrhocoris, Largus) they are absent as
far as known. So characteristic is the behavior of these chromo-
somes as to leave not the least doubt of their essential identity
throughout the whole series; and this series may be regarded as a
progressive one, in one direction or the other, with the same reason
as incase of any other graded series of morphological characters.
The series thus shown in case of the ra-chromosomes is as gradual
and complete as in case of the Y-chromosome, and may with
the same degree of probability be regarded as a descending one.
Thirdly, it is probable that the chromosome-number may
change by sudden mutations that produce extensive redistribu-
tions of the chromatin-substance without involving any commen-
surate change in its essential content. Were gradual changes,
chromosome by chromosome, the usual mode of modification,
we should certainly expect to find such conditions as are seen in
Nezara, in Notonecta, or in the Coreidae, more frequently. In
some groups, however, we find wide differences of chromosome-
number between species even of the same genus, and even be-
tween those that are very nearly related, without any accompany-
ing evidence of a gradual process of transition for instance,
among the aphids and phylloxerans (Stevens, Morgan) or in the
heteropterous genera Banasa and Thyanta. (Wilson, '09d.) In
Banasa dimidiata the diploid number is 16 in both sexes, in the
nearly related B. calva 26. Of the two races of Thyanta custa-
tor described above, apparently identical in other visible char-
acters, one has in both sexes the diploid number 16, with a
simple X-chromosome, while in the other the diploid number of
the male is 27 and that of the female 28, and the X-chromosome
consists of two components. It is improbable that the dif-
ferentiation of these two forms has been accomplished by grad-
ual modifications, chromosome by chromosome. It seems far
more likely that the change took place by sudden mutation, invol-
ving a redistribution of the nuclear material which changed its
form but not in an appreciable degree its substance. In the well
known case of Oenothera gigas, derived by sudden mutation from
Oe. Lamarckiana, a change by sudden mutation is known to be
104 EDMUND B. WILSON
PLATE 1
EXPLANATION OF FIGURES
All the figures from photographs of sections. Enlargement 1500 diameters.
10, 11 First spermatocyte-division (N. hilaris)
12, 13 The same (N. viridula)
14, 15 Second spermatocyte-division (hilaris)
16-25 Side views of second division (hilaris) . The XY-pair shown in 16-23, the
d-chromosome in 16, 17, 20, 24, 25; the small chromosome is evident in
10, 12, 13, 14, 15, 17, 18.
22 Initial separation of X and Y
23 Early anaphase, X and Y separating near the center (hilaris)
26-28 Nuclei from the growth-period, showing chromosome-nucleolus and plas-
mosome (hilaris)
29 Corresponding stage (viridula)
STUDIES ON CHROMOSOMES
EDMUND B. WILSON
PLATE 1
10
1 1
*
t
'4
" __^_^[HMME
'*'
tl '
18
20
!**
JOURNAL OF MORPHOLOGY, VOL. 22, NO. 1
STUDIES ON CHROMOSOMES 105
a fact (Lutz, '07; Gates, '08), though it may be due in this instance
to a simple doubling of the whole group. Such cases led me sev-
eral years ago to the conclusion "that the nucleus consists of
many different materials that segregate in a particular pattern
. . . and that the particular form of segregation may readily
change from species to species" (Wilson, '09d, p. 2).
Such changes must involve corresponding ones in the morpho-
logical and physiological value of the individual chromosomes;
and we must accordingly recognize the probability that these
individual values, though constant for the species, may change
from species to species as readily as does the number. Despite
the conformity to a general type often exhibited by particular
genera or even by higher groups, the individual chromosomes are
therefore per se of subordinate significance; and it may often be
practically impossible to establish exact homologies between those
of different species. How closely this bears on the origin of the
diverse conditions seen in the composition of the XY-pair is
obvious.
Lastly, it is almost certain that changes of number may some-
times arise as a result of abnormalities in the process of karyoki-
nesis, such as the passage of both daughter-chromosomes, or of
both members of a bivalent, to one pole. In Metapodius I found
('096) direct evidence of this in case of the XY-pair itself, and
endeavored to trace to this initial cause the remarkable variations
of number that occur in this genus. Many other observers have
recorded anomalies of this kind, in both plants and animals. It
seems entirely possible, as has been suggested by McClung ('05)
and by Gates ('08) that definite mutations may be traceable to this
cause; though probably such abnormalities may in general be
expected to lead to pathological conditions.
CONCLUSION
Some of the suggestions offered in the foregoing discussion are
admittedly of a somewhat speculative character; but they are not,
as I venture to think, mere a priori constructions, but are forced
upon our attention by the observed facts. The time has come
JOURNAL OF MORPHOLOGY, VOL. 22, NO. 1
106 EDMUND B. WILSON
when we must take into account more fully than has yet been done
the new complexities and possibilities that have continually been
unfolded as we have made better acquaintance with the chromo-
somes. In this respect the advance of cytology has quite kept
pace with that of the experimental study of heredity; and it has
established so close and detailed a parallelism between the two
orders of phenomena with which these studies are respectively
engaged as to compel our closest attention.
Studies on the chromosomes have steadily accumulated evi-
dence that in the distribution of these bodies we see a mechanism
that may be competent to explain some of the most complicated
of the phenomena that are being brought to light by the study
of heredity. Xew and direct evidence that the chromosomes
are in fact concerned with determination has been produced by
recent experimental studies, notably by those of Herbst ('09)
and Baltzer ( J 10) on hybrid sea-urchin eggs. But the interest
of the chromosomes for the study of heredity is not lessened, as
some writers have seemed to imply, if we take the view it is hi
one sense almost self-evident that they are not the exclusive
factors of determination. Through then" study we ma}' gain an
insight into the operation of heredity . that is none the less
real if the chromosomes be no more than one necessary link in a
complicated chain of factors. From any point of view it is
indeed remarkable that so complex a series of phenomena as is
displayed, for example, in sex-limited heredity can be shown to
run parallel to the distribution of definite structural elements,
whose combinations and recombinations can in some measure
actually be followed with the microscope. Until a better expla-
nation of this parallelism is forthcoming we may be allowed to hold
fast to the hypothesis, directty supported by so many other data,
that it is due to a direct causal relation between these structural
elements and the process of development.
A second point that may be emphasized is the remarkable con-
stancj' of the chromosome-relations in the species, and their no
less remarkable plasticity in the higher groups. The scepticism
that has been expressed in regard to constancy in the species finds,
I think, no real justification in the facts. It is perfectly true that
STUDIES OX CHROMOSOMES 107
individual fluctuations occasionally are seen in the number of the
chromosomes, in the process of synapsis, in the distribution of the
daughter-chromosomes, and hi all other cytological phenomena.
It is, however, also true that most observers who have made pro-
longed, detailed and comparative studies of any particular group,
have sooner or later reached the conviction that hi each species
all the essential relations in the distribution of the chromosomes
conform with wonderful fidelity to the specific type. So true is
this that the species may often at once be identified by an expe-
rienced observer from a single chromosome-group at any stage of
the maturation-process. No one, I believe, who has engaged for
a series of years in the detailed study of such a group, for instance,
as the Hemiptera or the Orthoptera, returning again and again to
the scrutiny of the same material, can be shaken hi the convic-
tion that the distribution of the chromosomes follows a perfectly
definite order, even though disturbances of that order now and
then occur. But it is equally important to recognize the fact
that this order has undergone many definite modifications of
detail from species to species, and that while all cases exhibit cer-
tain fundamental common features, we cannot without actual
observation predict the particular conditions hi any given case.
It is now evident that the larger groups vary materially in respect
to specific conditions. For instance, hi the orthopteran family of
Acrididae (McClung) the relations seem to be far more
uniform than such a group as the Hemiptera, where great spe-
cific diversity is exhibited, the details often changing from species
to species hi a surprising manner witness the species of Aphis
or Phylloxera (Stevens, Morgan), those of Acholla (Payne) or
of Thyanta (Wilson). In these respects, too, the cytologist finds
his experience running parallel to that of the experimenter on
heredity; and here, once more, we find it difficult not to believe
that both are studying, from different sides, essentially the same
problem.
December 13, 1910.
108 EDMUND B. WILSON
LITERATURE CITED
ARNOLD, G. 1908 The nucleolus and microchromosomes in the spermato-
genesis of Hydrophilus piceus. Arch. Zellforsch., vol. 2,
BALTZER 1909 Die Chromosomen von Strongylocentrotus lividus und Echinus
microtuberculatus. Arch. f. Zellforsch., Bd. 2.
1910 Ueber die Beziehung zwischen dem Chromatin und der Ent-
wicklung und Vererbungsrichtung bei Echinodermenbastarden. Habi-
litationsschrift, Wiirzburg. Engelmann, Leipzig.
BORING 1909 A small chromosome in Ascaris megalocephala. Arch., f. Zell-
forsch., vol. 4.
BOVERI, TH. 1909 " Geschlechtschromosomen" bei Nematoden. Arch. f. Zell-
forsch., Bd. 4.
BROWNE, E. N. 1910 The relation between chromosome-number and species
in Notonecta. Biol. Bull., vol. 20,1.
CASTLE, W. E. 1909 A Mendelian view of sex-heredity. Science, n. s., March 5.
COOK, M. H. 1910 Spermatogenesis in Lepidoptera. Proc. Acad. Nat. Sci.,
Philadelphia, April.
DBDERER, P. 1908 Spermatogenesis in Phyllosamia. Biol. Bull., vol. 13.
EDWARDS, C. L. 1910 Theidiochromosomesin Ascaris megalocephala and Ascaris
lumbricoides. Arch. f. Zellforsch., vol. 5.
GATES, R. R. 1908a The chromosomes of Oenothera. Science, n. s., vol. 27,
Aug. 2.
1908b A study of reduction in Oenothera rubrinervis. Bot. Gazette,
vol. 46,
1909 The behavior of the chromosomes in Oenothera lata x O. gigas.
Ibid., vol. 48.
GROSS, J. 1904 Die Spermatogenese von Syromastes marginatus. Zool. Jahrb.
Anat. u. Ontog., vol. 20.
GTJYER, M. 1910 Accessory chromosomes in man. Biol. Bull., vol. 19.
HERBST, C. 1909 Vererbungsstudien, VI. Die cytologischen Grundlagen der
Verschiebung der Vererbungsrichtung nach der mlitterlichen Seite.
Arch. Entwicklungsm., Bd., 27.
LUTZ, A. M. 1907 A preliminary note on the chromosomes of Oenothera La.
marckiana and one of its mutants. Sci., n. s. 26.
McCLUNG, C. E. 1905 The chromosome complex of orthopteran spermatocytes.
Biol. Bull., vol. 9.
STUDIES ON CHROMOSOMES 109
MONTGOMERY, T. H. 1901 A study of the chromosomes of Metazoa. Trans.
Am. Phil. Soc., vol. 20.
1906 Chromosomes in the spermatogenesis of the Hemiptera Heterop-
tera. Trans. Am. Phil. Soc., vol. 21.
MORGAN, T. H. 1900a A biological and cytological study of sex-determina-
tion in phylloxerans and aphids. Jour. Exp. Zool., vol. 7,
1910 Sex-limited inheritance in Drosophila. Science, n. s. 32, July 22.
MORIULL, C. V. 1910 The chromosomes in the oogenesis, fertilization and
cleavage of coreid Hemiptera. Biol. Bull., vol. 19.
PAULMIER, F. C. 1899 The spermatogenesis of Anasa tristis. Jour. Morph.,
vol. 15, Suppl.
PAYNE, F. 1909 Some new types of chromosome distribution and their rela-
tion to sex. Biol. Bull., vol. 16.
1910 The chromosomes of Acholla multispinosa. Biol. Bull., vol. 18.
RANDOLPH, HARRIET. 1908 On the spermatogenesis of the earwig, Anisolaba
maritima. Biol. Bull., vol. 15.
SINETY, R. DE 1901 Recherches sur la biologic et 1'anatomie des phasmes. La
Cellule, t. 19.
SPILLMAN, W. J. 1908 Spurious allemorphism. Results of some recent investi-
gations. Am. Naturalist, vol. 42.
STEVENS, N. M. 1906 Studies in spermatogenesis, II. A comparative study of
the heterochromosomes in certain species of Coleoptera, Hemiptera
and Lepidoptera, etc. Carnegie Inst. Pub. 36.
1908 A study of the germ-cells of certain Diptera, etc. Jour. Exp.
Zool., 5, 3.
1910 The chromosomes in the germ-cells of Culex. Jour. Exp. Zool.,
vo!8.
WALLACE, L. B. 1909 The spermatogenesis of Agalena nsevia. Biol. Bull.,
vol. 17.
WILSON, E. B. 1905a Studies on chromosomes, I. The behavior of the idiochro-
mosomes in Hemiptera. Jour. Exp. Zool., vol. 2.
1905b Studies on chromosomes, II. The paired microchromosomes,
idiochromosomes, and heterotropic chromosomes in Hemiptera. Jour.
Exp. Zool., vol. 2.
1906 Studies on chromosomes, III. The sexual differences of the
chromosomes in Hemiptera. Jour. Exp. Zool., vol. 3.
1909a Studies on chromosomes, IV. The accessory chromosome in
Syromastes and Pyrrhocoris. Jour. Exp. Zool., vol. 6.
110 EDMUND B. WILSON
1909b Studies on chromosomes, V. The chromosomes of Metapodius,
etc. Jour. Exp. Zool., vol. 6.
1909c Secondary chromosome-couplings and the sexual relations in
Abraxas. Science, n. s. 29, p. 748.
1909d Differences in the chromosome-groups of closely related
species and varieties, etc. Proc. Seventh Internat. Zool. Congress,
Aug. 1907.
1910a The chromosomes in relation to the determination of sex.
Science Progress, no. 16, April.
1910b Studies on chromosomes, VI. A new type of chromosome-com-
bination in Metapodius. Jour. Exp. Zool., vol. 9.
1910c Note on the chromosomes of Nezara. Science, n. s. 803, May 20.
C ~ I
Studies on Chromosomes
VIII. Observations on the Maturation-Phe-
nomena in Certain Heiniptera and Other
Forms, with Considerations on Synapsis
and Reduction
EDMUND B. WILSON
From the Depurt merit of /oology, Columbia University
from THK J U. WILSON
1'I.ATE 2
i r
33
34
35
37
43
*
* i: " i
38
I
39
45
;
46
THE JOUKNAI. OK KXPERIMENTAL ZOOLOGY, VOL. 13, NO. 3.
Wilson, Del.
435
PLATE 3
EXPLANATION OF FIGURES
Oncopeltus, from sections excepting 65, which is from a smear-preparation.
Enlargement 2250 diameters.
47-48 Spermatogonial telophases. It is uncertain whether this is the last divi-
sion or an earlier one.
49 Later spermatogonial telophase. Probably Stage a.
50-51 Stage b, showing the massive chromatic bodies, the sex-chromosomes
readily distinguishable.
52-55 Stages b-c, showing the process of unravelling.
56-59 Stage d. Leptotene-nuclei.
60 Transition to the synizesis. Synaptic period.
61 Stage e. Synizesis, from a very clear specimen.
62 Transition to the following stage.
63 Stage /. Pachytene-nucleus. The threads apparently undivided.
64 Stage /. Diplotene-nucleus.
65 (photo. 10). Stage/. Diplotene-nucleus, from a smear-preparation.
66-67 Stage g. The confused stage, showing plasmasome and both chromo-
some-nucleoli (sex-chromosomes). Plasmasome at its maximum size.
436
srrniKs ON CHROMOSOMES
KDMl'ND U. WILSON
PLATE 3
t I
48
49
50 51
.**
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54
55
59
60
63
64
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67
THE JOURNAL OK EXPERIMENTAL ZOOLOGY, VOL. 13, NO. 3
Hecljje, Del.
437
PLATE 4
EXPLANATION OF FIGURES
Lygaeus bicrucis (68-73, 83, 84), Largus cinctus (74-82), Anax junius (85-87),
Achurum (88-92). Enlargement 2250, excepting the figures of Achurum, which
are enlarged 1500 diameters.
68-70 Stage a. Earlier and later final spermatogonial telophases, from the
same cyst. Lygaeus.
71-72 Stage b. Lygaeus.
73 a-73 b Stage d. Leptotene-nuclei. Lygaeus.
74-75 Spermatogonial telophases, from the same cyst. Largus.
76-78 Stage c. Largus.
79-80 Stage d. Leptotene-nuclei. Largus.
81-82 Stage /. Diplotene-nuclei. Largus.
83-84 The same. Lygaeus.
85-87 Stages b-c. Anax.
88-92 Stages b-c. Achurum.
438
STUDIES ON CHROMOSOMES
EDMUND It. WILSON
PLATE 4
*
'36
74
-
/
75
IOUKNAI. OK EXI-KKIMKNTAI. ZOOLOGY t VOL. 13, NO. 3.
Hedge, Del.
439
PLATE 5
EXPLANATION OF FIGURES
Phrynotettix (93-96), Lygaeus (97-100), Largus (101-104), Orieopeltus (105-
108). From sections, excepting 107, 108, which are from smear-preparations.
The figures of Phrynotettix enlarged 1500 diameters, the others 2250 diameters.
93-95 Spermatogonial prophases of Phrynotettix. Fig. 93 is an early stage,
showing massive polarized bodies. The other figures show the uncoiling of the
spireme-threads from these bodies, 94 in side view (photo. 31), 95 as viewed from
the pole (photo. 30). Fig. 96 shows two successive stages lying side by side in the
same cyst (photo. 32).
97 The confused stage (Stage g) in Lygaeus.
98-99 Early prophases (Stage h) from Lygaeus.
100 a-h Isolated chromosome-nucleoli, from Stages/ and g in Lygaeus. showing
various forms of the sex-chromosomes assumed during the growth-period.
101-103 Largus cinctus. Nuclei transitional from Stage/ (diplotene) to Stage
g (confused period).
104 Nucleus of the confused period. Largus cinctus.
105-106 Early prophase-nuclei, Oncopeltus (early Stage h).
107 (photo. 17) Slightly later prophase-nucleus of Oncopeltus, showing early
bivalents.
108 (photo. 18) Later prophase of the same, early Stage i.
440
STUDIES ON CIIKOMOSOMKS
EDMUND B. WILhON
PLATE 5
101
103
vC
104
.
-
97
105
107
98
99
8
d e
f ff h
106
108
TUB JOURNAL OK EXl'EKIM ENTAI. ZOOLOGY, VOL. 13, NO. 3.
Hedge, Del.
441
PLATE 6
EXPLANATION OF FIGURES
From smear-preparations of Oncopeltus (109-114) and Protenor (115-120);
Enlargement 2250 diameters. (X designates the JT-chromosome, B the large
bivalent in Protenor, m, m, the m-chromosomes in the latter form.
109-114 Middle and late prophases of the first spermatocyte-division, showing
various forms of the bivalents during their condensation. In the earlier figures
the X- and F-chromosomes are short, longitudinally split rods (109-111); in the later
ones they are shortening to a dumb-bell form (112-114). Two of the same nuclei
are shown in photos. 20, 21.
115-117 Early prophases (Stage h), the bivalents just emerging from the con-
fused stage. The TO-chromosomes are but vaguely distinguishable.
118-119 Late Stage h, showing all the chromosomes, the bivalents still much
diffused.
120 (photo. 40) Nucleus from Stage i.
442
STUDIES ON CHROMOSOMES
EDMUND H. WILSON
PLATE 6
109
110
* '*
A
* *,,,
IHK JOURNAL OK EXfEKIM ENTAI. ZOOLUGY t VOL. 13, NO. 3.
Ifedj^e, Del.
443
PLATE 7
EXPLANATION OF FIGURES
From smear-preparations of Protenor; 2250 diameters; (lettering as in the pre-
ceding plate).
121-127 Middle prophases (Stage i) showing all the chromosomes. The m-
chromosomes, now condensed, are separate in all but 124. In 126 the large biva-
lent is a straight rod; in 127 it is such a rod bent at the middle point to form a V
(here seen edgewise so as not to show the longitudinal cleft. Some of the same
nuclei are shown in photos. 41 ; 44, 47.
128 (photo. 43) Abnormal nucleus in which the large bivalent is represented
by a pair of separate univalents (B, B) that have failed to unite in synapsis.
129-131 Late prophases (Stage j). In 131 the chromosomes are ready to enter
the inetaphase-plate; (fig. 131 also in photo. 51).
132-134 Early metaphases. In 132 one of the small bivalents and the m-chro-
mosomes appear abnormally large, owing to flattening. In 134 the ring tetrad
to the left has been slightly displaced in order to show it more clearly.
444
STt'DlKS OX CHROMOSOMES
EDMUND H. WILSON
PLATE 7
130
131
134
THE JOUKNAI. OK EXfEKIM KNTAI. ZOOLOGY, VOL. 13, NO. 3.
Hedge, Del.
PLATE 8
EXPLANATION OF FIGURES
From photographs by the author. Enlargement a little less than 1250 diameters.
Oncopeltus (1-5, 7-11, 16-24), Lygaeus bicrucis (6, 12-15, 25), Largus cinctus (26,
27). Many figures of the same preparations, from drawings, are reproduced in the
preceding plates, as indicated in brackets. Photos. 1-15, 26,27, from sections,
the others from smear-preparations.
1, 2 Spermatogonial metaphases. Oncopeltus.
3 First division metaphase.
4 Second division metaphase.
5 First division metaphase in side view, showing the sex-chromosomes in the
center.
6 (Figs. 45, 46) Second division metaphases, one in polar view, one in side
view, showing the initial separation of X and Y. Lygaeus bicrucis.
7-8 Stage d. Leptotene-nuclei of Oncopeltus.
9 Pachytene-nuclei, just emerging from the synizesis. Oncopeltus.
10 (Fig. 65). Stage/. Early diplotene-nucleus. From a smear.
11 Stage g. Confused stage, showing plasmasome and both sex-chromosomes.
12 Stage e. Synizesis, Lygaeus, from much extracted preparation, showing the
X- and F-chromosomes united.
13 Above, the X- and F-chromosomes of Lygaeus in Stage /, attached in one
case end to end, in the other side by side. Below, the same from early Stage g,
showing also the plasmasome.
14-15 Nuclei of Stage g, Lygaeus, showing the longitudinally divided X-chro-
mosome, and (at the left) the plasmasome.
16 Nucleus of the confused period (Stage g). Oncopeltus.
17-24 Early, middle and late prophases (Stages h-j) from smear-preparations
of Oncopeltus. Photo. 17 (fig. 107), 18 (fig. 108), 20 (fig. 109), 21 (fig. 111). The
sex-chromosomes distinguishable in each case.
25 Late prophase-nucleus of Lygaeus (Stage i-j), the X- and F-chromosomes
readily distinguishable above towards the left.
26-27 Stage b-c, in Largus cinctus. The uncoiling of spiral leptotene-threads
is clearly visible in the negative of photo. 27.
446
TII'IKS (>\ rlllHiMiiSO.MKS. VIII.
r.i'Mi M'
1'LATK s
Tin: .inriiNAi. "i K\CI:I:IMI:NT.M. EOdUtOT. vm.. 1:1 no ::
WILSON IMKiT'i
PLATE 9
EXPLANATION OF FIGURES
From photographs by the author. Enlargement as in the preceding plate.
Photos. 28-38 from sections (28-32 from McClung's preparations), 39-51 from
smear-preparations. Photo. 28, Achurum, 29-32 Phrynotettix, 33, 34 Largus cinc-
tus, 35-51 Protenor belfragei.
28 Stage c in Achurum. The unravelling threads clearly shown in the negative.
29 Early spermatogonial prophase of Phrynotettix, showing the massive chro-
matin-bodies just before the spiral thread is evident.
30 (Fig. 95). At the left, polar view of the coiled threads during the early
uncoiling, spermatogonial prophase.
31 (Fig. 94). The same stage (from a nucleus immediately adjoining in the
same section) seen in side-view.
32 (Fig. 96). Two adjoining nuclei, showing at the left an earlier, and at the
right a later stage of the uncoiling of the spireme-threads. The drawings of
these and the preceding photo, show threads at other levels as well.
33 Spermatogonial metaphase of Largus cinctus, 11 chromosomes, including
one large pair. The X-chromosome is one of the smaller ones, and can not be dis-
tinguished by the eye.
34 Metaphase of diploid group of the female of the same species, 12 chromo-
somes.
35 (Fig. 1 e, Wilson, '06). Spermatogonial metaphase of Protenor; 13 chromo-
somes.
36 The same. In both these photos, the large ^"-chromosome and the large
pair of autosomes are readily distinguishable.
37-38 Diploid chromosome-groups of the female Protenor, showing the -X"-pair
and the large pair of autosomes; 14 chromosomes.
39 Protenor. Above, a nucleus of the confused stage (g) showing the elongate
J-chromosome. Below are three final anaphases of the second division, showing
the passage of the undivided ^-chromosome to one pole.
40-51 Prophase-nuclei of Protenor. Photo. 40 (fig. 120), 41 (fig. 122) 43 (fig.
128), 44 (fig. 121), 46 (fig. 129), 47 (fig. 127), 51 (fig. 131).
448
>:
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THE JOURNAL OF EXPERIMENTAL ZOOLOGY,
VOLUME 13, NUMBER 3, OCTOBER, 1912
CONTENTS
Edmund B. Wilson
Studies on chromosomes. VIII. Observations on the matur-
ation-phenomena in certain Hemiptera and other forms,
with considerations on synapsis and reduction. v rom the
Department of Zoology, Columbia University. N ne plates 345
Wayland M. Chester
Wound closure and polarity in the tentacle of Metridium
marginatum. From The Museum of Comparative Zoology,
Harvard College. Eight figures 451
Max Morse
Artificial parthenogenesis and hybridization in the eggs of cer-
tain invertebrates. From Trinitv College . . 471
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