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Baws oe i len hy eat ee i ihn (ae eternal nea whee if : ae hae ae a cheats: Rete! : o si ahs Het aT i Hesrenicct: sane i i h i 7 11s wh aaa ase Hai! tay) a Nha Asiah Mtg la 4 vi! it ata hy ualhath nba fat ase Welt apa Raia aghast 4 a f ohtal Nya ties wh Pa ata an ih rd Stee eee ee , SiS anvildcanbedoushany ath onan isn a oN ah Pagdeaet eta it se aa gi Aha rp i er lite ee i ys aa geet) y i MAT x iA aby be a i Meulaqal Acaitg 5 4 y i i Pabse sadaae ie reds it i " A osirrer Mm Aipaetea 6 Ene te is : pats beh et ir estas “ reat! 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Veco hte ee pe Peatyh ppl Het er , rode ; ieey gett ast reies any West ese) salut rat eco) ; GEE bale iat Brciayas ayy wa Te re fiat 94 1e aig ts ny Rataey ae de Gygeas rar! +> athe! by! SH Hae 4g ty dle tar pr feet ia es Aa a] + aan athens baitsttae iy pnaieah et 0 ithe Pal rela ese itn fever tela Measite at my if or a i , Relat) y tsiene ay! eel Py fi pret ib ive } 4 Wht aees ade B Vepditdibal ib ERE et ue a UW iain He é ash a ithyas 9 ee sUarabaeapsteistet aecatat dheash aay area rt dh ge con nes HL palais ie pant a fet sot abound atta mab preg, ep arena sein , 4 tit thy etal g igi hal i bans z Mardy a as PHL bast pou ee ca rh Ub rol: Ba eS tr! sume ta é iguat at poncgaaiar atti Ni THE UNIVERSITY OF ILLINOIS LIBRARY Be Sy M8abh as Return this book on or before the Latest Date stamped below. A charge is made on all overdue books. | University of Illinois Library OV 30 1955 | F Ef 09 5p, 629 03° MAY 23 1951 iy MAY 0 3 1994 Digitized by the Internet Archive in 2021 with funding from University of Illinois Urbana-Champaign https://archive.org/details/hereditysexOOmorg_0 Columbia Enibersity Lectures HERED TY AND SEX THE JESUP LECTURES 1913 COLUMBIA UNIVERSITY PRESS SALES AGENTS NEW YORK: LEMCKE & BUECHNER 380-32 West 27TH STREET LONDON: HUMPHREY MILFORD AMEN CorRNneER, E.C. TORONTO: HUMPHREY MILFORD 25 Ricumonp 8t., W. COLUMBIA UNIVERSITY LECTURES HEREDITY AND SEX BY. BEOMAS HUNT MORGANS baad: PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY Nets Work COLUMBIA UNIVERSITY PRESS 1913 All rights reserved CopyRiGuT, 1913, By COLUMBIA UNIVERSITY PRESS. Set up and electrotyped. Published November, 1913. Norwood jpress J. 8S. Cushing Co. — Berwick & Smith Co, Norwood, Mass., U.S.A, cv INTRODUCTION Two lines of research have developed with surpris- ing rapidity in recent years. Their development has been independent, but at many stages in their progress they have looked to each other for help. The study of the cell has furnished some fundamental facts connected with problems of heredity. The modern study of heredity has proven itself to be an instrument even more subtle in the analysis of the materials of the germ-cells than actual observations on the germ- cells themselves. In the following chapters it has been my aim to point out, wherever possible, the bearing of cytological studies on heredity, and of the study of heredity on the analysis of the germinal materials. The time has come, I think, when a failure to recog- nize the close bond between these two modern lines of advance can no longer be interpreted as a wise or cautious skepticism. It seems to me to indicate rather a failure to appreciate what is being done at present, and what has been accomplished. It may not be desir- able to accept everything that is new, but it is cer- tainly undesirable to reject what is new because of its newness, or because one has failed to keep in touch with the times. An anarchistic spirit in science does not always mean greater profundity, nor is our attitude toward science more correct because we are unduly e ye omy S4 9729 vl INTRODUCTION skeptical toward every advance. Our usefulness will, in the long run, be proven by whether or not we have been discriminating and sympathetic in our attitude toward the important discoveries of our time. While every one will probably admit such generalities, some of us may call those who accept less than ourselves con- servatives; others of us who accept more will be called rash or intemperate. To maintain the right balance is the hardest task we have to meet. In attempting to bring together, and to interpret, work that is still in the making I cannot hope to have always made the right choice, but I may hope at least for some indulgence from those who realize the difficulties, and who think with me that it may be worth while to make the attempt to point out to those who are not specialists what specialists are thinking about and doing. What I most fear is that in thus attempting to for- mulate some of the difficult problems of present-day interest to zodlogists I may appear to make at times unqualified statements in a dogmatic spirit. I beg to remind the reader and possible critic that the writer holds all conclusions in science relative, and subject to change, for change in science does not mean so much that what has gone before was wrong as the discovery of a better strategic position than the one last held. TABLE OF CONTENTS INTRODUCTION CHAPTER I THE EVOLUTION OF SEX 1. REPRODUCTION, A DISTINCTIVE FEATURE OF LIVING THINGS b THE “MEANING” OF SEXUAL REPRODUCTION THE Bopy AND THE GERM-PLASM THE EaAarty ISOLATION OF THE GERM-CELLS iets ee oS THE APPEARANCE OF THE ACCESSORY ORGANS OF REPRODUCTION 6. THE SECONDARY SEXUAL CHARACTERS 7. THe SEXvuAL INSTINCTS CHAPTER II THE MECHANISM OF SEX-DETERMINATION 1. THe MATURATION OF THE EGG AND THE SPERM bo TuHE CYTOLOGICAL EVIDENCE a. Protenor b. Lygeus c. Oncopeltus . d. Ascaris : ; e. Aphids and Phylloxerans 3. THE EXPERIMENTAL EVIDENCE a. The Experiments on Sea-urchins’ Eges b. The Evidence from Sex-linked Inheritance vii PAGES . v-vl 1-4 » 4-15 15-19 20-23 25-26 26-31 31-34 vill TABLE OF CONTENTS CHAPTER III THE MENDELIAN PRINCIPLES OF HEREDITY AND THEIR BEARING ON SEX ’ PAGES 1. MENDEL’s DISCOVERIES . : . ; 4 ; 73-715 2. THe HEREDITY OF ONE PAIR OF CHARACTERS . ‘ 75-80 3. THE HEREDITY OF A SEX-LINKED CHARACTER . ; 80-84 4. Tor Herepity oF Two Parrs or CHARACTERS ; 84-88 5. THe HerreEpitry OF Two SEX-LINKED CHARACTERS . 88-93 6. A THEORY OF LINKAGE : : ; : ? ‘ 93-97 7. THREE SEX-LINKED FaAcTors ; ; ; ‘ ; 98-100 CHAP THER OLY SECONDARY SEXUAL CHARACTERS AND THEIR RELATION TO DARWIN’S THEORY OF SEX- UAL SELECTION 1. THe OccuRRENCE OF SECONDARY SEXUAL CHARAC- TERS IN THE ANIMAL KINGDOM . : : . 101-112 2. CouRTSHIP : ; : : : ; : ‘ . 112-120 3. VIGOR AND SECONDARY SEXUAL CHARACTERS . . 120-121 4, ConTINUOUS VARIATION AS A BASIS FOR SELECTION 121-125 5. DISCONTINUOUS VARIATION OR MUTATION AS A BASIS FOR SELECTION : ; : : ‘ ; . 125-131 CHAPTER V THE EFFECTS OF CASTRATION AND OF TRANS- PLANTATION ON THE SECONDARY SEXUAL CHARACTERS 1. OPERATIONS ON MAMMALS . , . : ‘ . 132-141 2. OPERATIONS ON BirpDs . : 3 , : ; . 142-144 3. OPERATIONS ON AMPHIBIA . : é : 4 . 145-146 4. INTERNAL SECRETIONS . ‘ : : : . . 146-147 5. OPERATIONS ON INSECTrs : : : . y . 148-155 6. PARASITIC CASTRATION OF CRUSTACEA ; ; . 155-158 TABLE OF CONTENTS CHAPTER VI 1X GYNANDROMORPHISM, HERMAPHRODITISM, PARTHENOGENESIS, AND SEX 1. GYNANDROMORPHISM 2, HERMAPHRODITISM. 3. PARTHENOGENESIS . 4. ARTIFICIAL PARTHENOGENESIS CHAPTER VII FERTILITY 1. INBREEDING CROSS-BREEDING bo SEXUAL REPRODUCTION IN PARAMGCIUM sa THEORIES OF FERTILITY CHAPTER VIII SPECIAL CASES OF SEX-INHERITANCE 1. Sex in BEEs 2. A SEX-LINKED LETHAL FACTOR 3. NON-DISJUNCTION OF THE SEX-CHROMOSOMES 4. Tur VANISHING MALES OF THE NEMATODES 5. SEX-RATIOS IN HyBrip BIRDS AND IN CROSSED Races IN MAn 6. SEX-RATIOS IN FROGS 7. SEX-RATIOS IN MAN 8. THE ABANDONED VIEW THAT EXTERNAL CONDITIONS DETERMINE SEX 9. SEX-DETERMINATION: IN MAN BIBLIOGRHA PHY INDEX PAGE 161-167 167-175 173-188 188-195 194-199 200-207 207-211 211-219 220-221 221-223 bo BS ob bs ery % 3 bo teat en) nm 5) 2-236 36-2 49 bo bo 25ISBFS 279-282 HEREDITY AND SEX Gap ere heel THE EVOLUTION OF SEX ANIMALS and plants living to-day reproduce them- selves in a great variety of ways. With a modicum of ingenuity we can arrange the different ways in series beginning with the simplest and ending with the more complex. In a word, we can construct systems of evolution, and we like to think that these systems reveal to us something about the evolutionary process that has taken place. There can be no doubt that our minds are greatly impressed by the construction of a graded series of stages connecting the simpler with the complex. It is true that such a series shows us how the simple forms might conceivably pass by almost insensible (or at least by overlapping) stages to the most complicated forms. This evidence reassures us that a process of evolution could have taken place in the imagined order. But our satisfaction is superficial if we imagine that such a survey gives much insight either into the causal processes that have produced the successive stages, or into the interpretation of these stages after they have been produced. Such a series in the present case would culminate in a process of sexual reproduction with males and 1 2 HEREDITY AND SEX females as the acters in the drama. But if we are asked what advantage, if any, has resulted from the process of sexual reproduction, carried out on the two-sex scheme, we must confess to some un- certainty. The most important fact that we know about living matter is its inordinate power of increasing itself. If all the fifteen million eggs laid by the conger eel were to grow up, and in turn reproduce, in two years the sea would be a wriggling mass of fish. A single infusorian, produced in seven days 935 de- scendants. One species, stylonichia, produced in 64% days a mass of protoplasm weighing one’ kilogram. At the end of 30 days, at the same rate, the number of kilograms would be 1 followed by 44 zeros, or a mass of protoplasm a million times larger than the volume of the sun. Another minute organism, hydatina, produces about 30 eggs. At the end of a year (65 generations), if all the offspring survived, they would form a sphere whose limits would extend beyond the confines of the known universe. The omnipresent English sparrow would produce in 20 years, if none died except from old age, so many de- scendants that there would be one sparrow for every square inch of the State of Illinois. Even slow- breeding man has doubled his numbers in 25 years. At the same rate there would in 1000 years not be standing room on the surface of the earth for his offspring. I have not gone into these calculations and will THE EVOLUTION OF SEX 3 not vouch for them all, but whether they are en- tirely correct or only partially so, they give a rough idea at least of the stupendous power of growth. There are three checks to this process: First, the food supply is insufficient — you starve; second, ani- mals eat each other — you feed; third, substances are produced by the activity of the body itself that inter- fere with its powers of growth — you poison yourself. The laws of food supply and the appetites of enemies are as inexorable as fate. Life may be defined as a constant attempt to find the one and avoid the other. But we are concerned here with the third point, the methods that have been devised of escape from the limitations of the body itself. This is found in repro- duction. The simplest possible device is to divide. This makes dispersal possible with an increased chance of finding food, and of escaping annihilation, and at the same time by reducing the mass permits of a more ready escape of the by-products of the living machine. Reproduction by simple division is a well-known pro- cess in many of the lower animals and plants; it is almost universal in one-celled forms, and not unknown even in many-celled organisms. Amoeba and _ para- moecium are the stock cases for unicellular animals; many plants reproduce by buds, tubers, stolons, or shoots; hydroids and sea-anemones both divide and bud; many planarians, and some worms, divide trans- versely to produce two new individuals. But these methods of reproduction are limited to simple structures where concentration and division of labor amongst the organs has not been carried to an extreme. In con- sequence, what each part lacks after the division can be 4 HEREDITY AND SEX quickly made good, for delay, if prolonged, would increase the chances of death. But there is another method of division that is almost universal and is utilized by high and by low forms alike : individual cells, as eggs, are set free from the rest of the body. Since they represent so small a part of the body, an immense number of them may be produced on the chance that a few will escape the dangers of the long road leading to maturity. Sometimes the eggs are protected by jelly, or by shells, or by being trans- parent, or by being hidden in the ground or under stones, or even in the body of the parent. Under these circumstances the animal ventures to produce eggs with a large amount of food stored up for the young embryo. So far reaching were the benefits of reproduction by eggs that it has been followed by almost every species in the animal and plant kingdom. It is ad- hered to even in those cases where the animals follow other grosser methods of separation at the same time. We find, however, a strange limitation has been put upon the process of reproduction by eggs. Before the ege begins its development it must be fertilized. Cells from two individuals must come together to produce a new one. The meaning of this process has baffled biologists ever since the changes that take place during fertili- zation were first discovered; in fact, long before the actual processes that take place were in the least un- derstood. There is a rather extensive and antiquated literature dealing with the part of the male and of the female in the process of procreation. It would take us too far to attempt to deal with these questions THE EVOLUTION OF SEX 3) in their historical aspects, but some of their most modern aspects may well arrest our attention. In the simplest cases, as shown by some of the one- celled organisms, two individuals fuse into a single one (Fig. 1); in other related organisms the two in- dividuals that fuse may be unequal in size. Some- times we speak of these as male and female, but it is questionable whether we should apply to these unicellular types the same names that we use for the Fia. 1. — Union of two individuals (Stephanosphera pluvialis) to form a single individual. (After D6flein.) many-celled forms where the word sex applies to the soma or body, and not to the germ cells. One of the best known cases of conjugation is that of paramoecium. Under certain conditions two in- dividuals unite and partially fuse together. An in- terchange of certain bodies, the micronuclei, then takes place, as shown in Fig. 2, and in diagram, Fig. 3. The two conjugating paramoecia next separate, and each begins a new cycle of divisions. Here each individual may be said to have fertilized the other. The process recalls what takes place in hermaphroditic animals of higher groups in the sense that sperm from one indi- vidual fertilizes eggs of the other. We owe to Maupas the inauguration of an epoch- making series of studies based on phenomena like this in paramcecium. 6 HEREDITY AND SEX Fig. 2. — Conjugation in Paramcecium. The micronucleus in one indi- vidual is represented in black, in the other by cross-lines. The macro- nucleus in both is stippled. A—C, division of micronucleus into 2 and 4 nuclei; C'!—D, elongation of conjugation nuclei, which interchange and recombine in #; F—J/, consecutive stage in one ex-conjugant to show three divisions of new micronucleus to produce eight micronuclei (J). In lower part of diagram the first two divisions of the ex-conjugant (J) with eight micronuclei are shown, by means of which a redistribution of the eight micronuclei takes place, See also Fig. 100. “I THE EVOLUTION OF SEX Fic. 3. — The nuclei of two individuals of paramcecium in I (homozygous in certain factors, and heterozygous in other factors), are represented as divid- ing twice (in II and III); the first division, II, is represented as reducing, i.e. segregation occurs; the second division, III, is represented as equational, z.e. no reduction but division of factors, as in the next or conjugation division, IV, also. 8 HEREDITY AND SEX Maupas found by following from generation to generation the division of some of these protozoa that the division rate slowly declines and finally comes to an end. He found. that if a debilitated individual conjugates with a wild individual, the death of the race is prevented, but Maupas did not claim that through conjugation the division rate was restored. On the contrary he found it is lower for a time. He also discovered that conjugation between two related individuals of these weakened strains produced no beneficial results. Biutschli had earlier (1876) suggested that conjugation means rejuvenation or renewal of youth, and Maupas’ results have sometimes been cited as supporting this view. Later work has thrown many doubts on this interpretation and has raised a number of new issues. In the first place, the question arose whether the decline that Maupas observed in the rate of division may not have been due to the uniform conditions under which his cultures were maintained, or to an insuffi- ciency in some ingredient of these cultures rather than to lack of conjugation. Probably this is true, for Calkins has shown that by putting a declining race into a different medium the original division rate may be restored. Woodruff has used as culture media a great variety of food stuffs and has succeeded in keep- ing his lines without loss of vigor through 3000 gen- erations. Maupas records a decline in other related protozoa at the end of a few hundred generations. Biitschhl’s idea that by the temporary union (with interchange of micronuclei) of two weak individuals two vigorous individuals could be produced seems THE EVOLUTION OF SEX 9 mysterious; unless it can be made more explicit, it does not seem in accord with our physico-chemical conceptions. Jennings, who has more recently studied in greater detail the process of division and conjugation in paramcecium, has found evidence on which to base a more explicit statement as to the meaning of rejuve- nescence through conjugation. Jennings’ work is safeguarded at every turn by care- ful controls, and owing in large part to these controls his results make the interpretations more certain. He found in a vigorous race, that conjugated at rather definite intervals, that after conjugation the division rate was not greater than it had been before, but on the contrary was slower — a fact known, as he points out, to Maupas and to Hertwig. Conjugation does not rejuvenate in this sense. Jennings states that, since his race was at the be- ginning vigorous, the objection might be raised that the conditions were not entirely fulfilled, for his pred- ecessors had concluded that it is a weakened race that was saved from annihilation by the process. In order to meet this objection he took some individuals from his stock and reared them in a small amount of culture fluid on a slide. After a time they became weakened and their rate of division was retarded. He then al- lowed them to conjugate, and reared the conjugants. Most of these were not benefited in the least by the process, and soon died. A few improved and began to multiply, but even then not so fast as paramcecia in the control cultures that had been prevented from con- jugating. Still others gave intermediate rates of division. 10 HEREDITY AND SEX He concludes that conjugation is not in itself bene- ficial to all conjugants, but that the essence of the pro- cess 1s that a recombination of the hereditary traits occurs as shown in the diagram, Fig. 3 and 4. Some 5 ee ee ‘AB | a A°D Be Aa Cals =e a BY au Nin S ( CD Fic. 4. — Illustrating conjugation between two stocks, with pairs of factors A, B, C, D, and a, b, c,d; and union of pairs into Aa, Bb, Cc, Dd. After these separate, their possible recombinations are shown in the 16 smaller circles. (After Wilson.) of these new combinations are beneficial for special conditions — others not. The offspring of those con- jugants that have made favorable combinations will soon crowd out the descendants of other conjugants that have made mediocre or injurious combinations. Hence, in a mass culture containing at all times large THE EVOLUTION OF SEX 11 numbers of individuals, the maximum division rate is kept up, because, at any one time, the majority of the individuals come from the combinations favorable to that special environment. There are certain points in this argument that call for further consideration. In a mass culture the fa- vorable combinations for that culture will soon be made, if conjugation is taking place. At least this is true if such combinations are homogeneous (homozygous, in technical language). Under such circumstances the race will become a pure strain, and further conjugation could do nothing for it even if it were transferred to a medium unsuited to it. In the ordinary division of a cell every single de- terminer divides and each of the new cells receives half of each determiner. Hence in the case of para- moecium all the descendants of a given paramoecium that are produced by division must be exactly alike. But in preparation for conjugation a different pro- cess may be supposed to take place, as in higher animals, among the determiners. The determiners unite in pairs and then, by division, separate from each other, Fig. 4. In consequence the number of determiners is reduced to half. Each group of deter- miners will be different from the parent group, pro- vided the two determiners that united were not identical. If after this has occurred conjugation takes place, the process not only restores the total number of determiners in each conjugant, but gives new groups that differ from both of the original groups. The maintenance of the equilibrium between an 12 HEREDITY AND SEX organism and its environment must be a very delicate matter. One combination may be best suited to one environment, and another combination to another. Conjugation brings about in a population a vast num- ber of combinations, some of which may be suited to the time and place where they occur. ‘These survive and produce the next generation. Jennings’ experiments show, if I understand him correctly, that the race he used was not homogeneous in its hereditary elements; for when two individuals conjugated, new combinations of the elements were formed. It seems probable, therefore, that the chemi- eal equilibrium of paramcecium is maintained by the presence of not too much of some, or too little of other, hereditary materials. In a word, its favorable com- binations are mixed or heterozygous. The meaning of conjugation, and by implication, the meaning of fertilization in higher forms is from this point of view as follows :— In many forms the race, as a whole, is best maintained by adapting itself to a widely varied environment. z na Ny | spermatozoa. These two classes of spermatozoa are present in equal numbers. If we study the body cells of the female protenor, we find fourteen chromosomes (Fig. 22, A). Twelve of these are the ordinary chromosomes, and two, larger than the rest, are the sex chromosomes. At the synap- sis stage all of the chromosomes unite in pairs, including the two sex chromosomes. When the process is finished, there are seven double chromosomes (Fig. 22, B). THE MECHANISM OF SEX-DETERMINATION 43 When the egg sends off its two polar bodies, the chro- mosomes divide or separate. At the first division seven chromosomes pass out (C), and seven remain in the egg. At the next division the seven chrcemosomes in the egg divide again, seven pass out and seven remain Prolerior a) HrGs e228 = in the egg (D). Of these seven, one chromosome, recognizable by its large size, is the sex chromosome. All the eggs are alike (#). There is only one kind of egg, but there are two kinds of sperm. Any egg that is fertilized by a sperm carrying six chromosomes pro- duces an individual with thirteen chromosomes. This individual is a male. Any egg that is fertilized by a sperm carrying seven 4 HEREDITY AND SEX chromosomes produces an individual with fourteen chromosomes. ‘This individual is a female. In another species of insect, Lygzeus bicrucis, the male differs from the female, not in having a different Lygaeus OC @ o®@ YIN i & \ @ 40808 Ai. 2@ Se @@o e rs \/ D ae W INQ o* \ Hl ‘ts : | mh *< GS r aw Hi A, ae iy @e0°800 Ye number of chromosomes as in protenor, but by the occurrence of a pair of different-sized chromosomes. The body cells of the male have twelve ordinary chromosomes and two sex chromosomes — one larger, xX than: thesotherwy 3 hie.23) Ag: After synapsis there are six double chromosomes and the two sex chromosomes, called X and Y (Fig. 23, D). BIGe 223% THE MECHANISM OF SEX-DETERMINATION 45 At the first spermatocyte division all the chromosomes divide (C). The two resulting cells have eight chro- mosomes, including X and Y. At the second division (D) the double chromosomes again divide, but X and Y do not divide. They approach and touch each other, and are carried into the spindle, where they separate from each other when the other ordinary chromosomes Lygaeus © divide. Consequently there are formed two kinds of spermatozoa — one containing X and the other Y (Fig. 23, E). In the body cells and early germ-tract of the female of lygeus (Fig. 24, A), there are twelve ordinary chromosomes and two sex chromosomes, X and X. After reduction there are seven double chromosomes, the two X’s having united when the other chromosomes 46 HEREDITY AND SEX united (B). Two divisions take place (C, D), when the two polar bodies are formed, leaving seven chromosomes in the egg (#). Each egg contains as a result only one X chromosome. Any egg of lygeeus fertilized by a sperm carrying an X chromosome produces a female that contains two S _ Ady @ee @° ee e*°e —— @0000eee @0ee@ ee sphere ote Gee te A J \ Ni E eee &o3— Soee°'see bs Y \\\\ i // fl \\ / / | | i z \\ Yn Oe ral Ali sa \ \ e0ee088e os bad YM LZ Cc Nl a Et or. linge, YaGy, EZ Oncopellus 3 X’s or XX. Any egg fertilized by a sperm containing a Y chromosome produces a male that contains one X and one YsorexX Yo Another insect, Oncopeltus fasciatus, represents a third type in which the chromosome groups in the male and in the female are numerically alike and alike as to visible size relations. THE MECHANISM OF SEX-DETERMINATION = 47 In the body cells of the male there are sixteen chro- mosomes (Fig. 25, A). After reduction there are nine chromosomes — seven in a ring and two in the middle (B). The seven are the fused pairs or double chro- mosomes; the two in the middle are the sex chromo- somes that have not fused. Oncopettus ¢ Fig. 26. The evidence for this interpretation is circumstan- tial but sufficient. At the first reduction division all nine chromosomes divide (C). Just before the second division the two central chromosomes come together and remain in contact (DD’). All the double chromosomes then divide, while the two sex chromosomes simply sepa- rate from each other, so that there are eight chromo- somes at each pole (DE). 3 48 HEREDITY AND SEX In this case all of the spermatozoa (HE’) contain eight chromosomes. There is no visible difference between them. Nevertheless, there is reason for be- lieving that here also there are two kinds of sperm. The principal reason 1s that, there are all connecting stages between forms in which there is an unequal pair, Ascaris 3 ve US 7 ‘A i WW be Ai LYN® eS B \\ as in lygeeus, and forms with an equal pair, as in oncopel- tus. Another reason is that the two sex chromosomes behave during the synapsis stages as do the X Y chromo- somes in related species. Moreover, the experimental evidence, of which I shall speak later, leads us to con- clude that the determination of sex is not due only to THE MECHANISM OF SEX-DETERMINATION 49 a difference in size of X and Y. The sex chromosomes must carry a host of factors other than those that de- termine sex. Consequently it is not surprising that in many species the sex chromosomes appear equal or nearly equal in size. It is a fortunate circumstance for us that in some species there is a difference in size or AsCcarts 9 NS wd) Fig. 28. an unpaired sex chromosome; for, in consequence, we are able to trace the history of each kind of sperm in these cases; but it is not essential to the theory that X and Y, when present, should be visibly different. In the female of oncopeltus sixteen chromosomes occur as in the male (Fig. 26, 4). The reduced number is eight double chromosomes (B). At one of the two polar divisions eight chromosomes pass out, and eight remain in the egg (C). At the second division also eight pass out, and eight remain in the egg (D). 00 HEREDITY AND SEX I shall pass now to a fourth condition that has only recently come to light. It is best shown in some of the nematode worms, for example, in the ascaris of the horse. Here the sex chromosomes are generally at- tached to other chromosomes. In this case, as shown by the diagram (Fig. 27, A), there is in the male a single X attached to one of the other chromosomes. At the first spermatocyte division it does not divide (C), but passes over bodily to one pole, so that two kinds of cells are produced. At the second spermatocyte division it divides, in the cell that contains it, so that each daughter cell gets one X (D). Two classes of sperm result, two with X (//), two without (H’). In the female there are two X’s, each attached to a chromosome (Fig. 28). After the polar bodies are given off, one X only is left in each egg (C, D, FH). Sex is determined here in the same way as in the insects, described above, for there are two classes of sperm and but one class of eggs. The discovery of the sex chromosome and its rela- tion to sex is due to several investigators. In 1891 Henking first described this body, and its unequal distri- bution, but was uncertain even as to its relation to the chromosomes. Paulmier (1899), Montgomery (1901), Sinéty (1901), gave a correct description of its behavior in spermatogenesis. McClung (1902) confirmed these discoveries, and suggested that the accessory, or odd chromosome, as it was then called, had some relation to sex, because of its unequal distribution in the sperms. He inferred that the male should have one ~more chromosome than the female, but he gave no evi- dence in support of this suggestion, which as we have THE MECHANISM OF SEX-DETERMINATION dl seen is the reverse of the actual conditions. Stevens (1905) made out the relations of the X Y pair of chro- mosomes to sex and Wilson in the same year (1905) the correct relation of the accessory chromosome to sex. The results described above for the insects are for the most part from Wilson’s studies on the chromosomes ; these for ascaris from the recent work of Sophia Frolowa, which confirms in the main the work of Boveri, Gulick, Boring, and Edwards. A case similar to ascaris has been described by Stevens for the mosquito, in which there is an XY and a Y in the male, each attached to another chromosome. In the guinea pig also, there seems to be in the male an X and a Y, attached to another pair of chromosomes. Find- ing these cases so widely distributed, it seems not un- likely that in other cases, where an unpaired X or an X and a Y have not been detected, they are parts of other chromosomes. The whole history of the sex chromosomes of ancyro- canthus, a nematode worm, is strikingly shown in a recent paper by Carl Mulsow (Fig. 29 and 29a, A). This is a typical case in which the male has one less, chromosome than the female, as in protenor. The case is striking because the chromosomes can be seen and counted in the living spermatozoa. Some sperm have six, some have five chromosomes. The sperm- nucleus can be identified in the egg after fertilization because it lies nearer the pole opposite to the polar bodies. The entering sperm nuclei show in half of the fertilized eggs six chromosomes and in the other half five chromosomes. An interesting confirmation of these conclusions in o2 HEREDITY AND SEX regard to the relation between sex and the sex chromo- somes was found in another direction. It has long been known that the fertilized eggs of aphids or plant lice produce only females. The same thing happens in near relatives of the plant lice, the phylloxerans. ot ° X - e . CN ey os h ? ?* Pe fons “ : “.@ eset . ee S of, ¢ Ad e* ity, # “ " ee - ‘ 8 ae © bead * ee * : °° a OF: “ee xs e eof ‘ Ps y ° a @ ° ’ mie 5 3 “eo Ped * Dy ‘ast ‘on se . - 2s “i &, s . et ee 4 % <7 { we i, Ay P| ‘ rh 4 2 bd +e ber) e +? fee a 1” Fic. 29.—1 and 2 are spermatogonia; 3, growth period; 4-7, prophases; 8, equatorial plate of first division, 9-10; 11, spermatocytes of second order ; 12-13, division of same; 14-16, the four cells or spermatids that come from the same original cell, two with 5, two with 6 chromosomes; 17, spermatids; 18, mature sperm; 19, living sperm. (After Mulsow.) In these insects a study of the chromosomes shows that the male has one less chromosome than the female. At the first maturation division in the male (Fig. 30), all the chromosomes divide except one, the X chromo- some, and this passes to one cell only. This cell is also larger than the sister cell. The small cell lacking the X degenerates, and does not produce spermato- THE MECHANISM OF SEX-DETERMINATION — 53 zoa. The large cell divides again, all of the chromo- somes dividing. Two functional spermatozoa are produced, each carrying one sex chromosome. These spermatozoa correspond to the female-producing sper- matozoa of other insects. In the sexual female there is an even number of chro- Fig. 29a. — 20 and 21, odgonia (equatorial plate); 22, growth period; 23, before fertilization; 24-25, entrance of sperm; 26-31, prophases of first division; 32-33, formation of first polar body; 34-36, extrusion of same and formation of second polar body; 37, two pronuclei; 38-41, union of pronuclei; 42-45, cleavage. (After Mulsow.) mosomes — one more than in the male. They unite in pairs. When the two polar bodies of the sexual ege are formed, all the chromosomes divide twice, so that each egg is left with one sex chromosome. It is now evident why only females are produced after fertilization. The female-producing sperm alone is functional. 54 HEREDITY AND SEX Second Spermatocyle: O_O @) Wem. Fiag. 30.— Diagram of chromosomes in Phylloxera caryecaulis. Top line, somatic cell of female with 6 chromosomes and somatic cell of male with 5 chromosomes. Second line, stages in first spermatocyte division producing a rudimentary cell (below) with two chromosomes. Third line, second spermatocyte division into two equal cells. Fourth line, sexual egg (3 chromosomes) and two polar bodies; and two functional, female- producing sperm with three chromosomes each. © 0 6) Fitst wWShermaloeyl, @ @ 22) THE MECHANISM OF SEX-DETERMINATION 59 THE EXPERIMENTAL EVIDENCE The experimental evidence, indicating that there is an internal mechanism for sex determination, is derived: from two sources — from experimental embryology, and from a study of the heredity of sex-linked characters. The evidence from embryology shows that the chro- mosomes are the bearers of materials essential for the production of characters. The evidence from hered- ity shows that certain characters follow the sex chromosomes. It has long been taught that the hereditary factors are carried by the nucleus. The evidence for this was found in fertilization. When the spermatozo6n enters the egg, it carries in, as a rule, only the head of the sper- matozoon, which consists almost entirely of the nucleus of the original cell from which it comes. Since the male transmits his characters equally with the female, it follows that the nucleus is the source of this inheritance. The argument has not been regarded as entirely conclusive, because the sperm may also bring in some of the protoplasm of the original cell—at least that part lying immediately around the nucleus. In addition a small body lying at the base of the sperm head seems also to be brought in by the male, and according to some observers it becomes the center about which the entire division system or karyokinetic spindle develops. The most convincing evidence that the chromosomes are the most important elements in heredity is found in some experimental work, especially that of Boveri, Baltzer, and Herbst. Under certain circumstances in 56 HEREDITY AND SEX the sea-urchin two spermatozoa may enter a single egg. They both unite with the egg nucleus (Fig. 31). Each brings in 18 chromosomes. ‘The egg contributes 18 chromosomes. There are in all 54, instead of 36 chromosomes, as in normal fertilization. Fic. 31. — Dispermy and its effects in egg of sea urchin. (After Boveri.) Around these chromosomes a double system of threads develops with four poles. The chromosomes become unequally distributed on the four spindles that develop. Each chromosome then divides, and half of each goes to the nearest pole. To some of the poles many chromosomes may pass, to other poles fewer. THE MECHANISM OF SEX-DETERMINATION ‘57 In order to simplify the case let us imagine that each sperm has only four chromosomes and the egg nucleus only four. Let us represent these by the letters as shown in Fig. 32. Any one of the four cells that is Fig. 32. — Diagram illustrating the irregular distribution of the chro- mosomes in dispermic eggs in an imaginary case with only four kinds of chromosomes, a, b, c, d. There are here three sets of each of these in each egg. The stippled cells are those that fail to receive one of each kind of chromosome. (After Boveri.) produced at the first division of these dispermic eggs may contain a full complement of the chromosomes, or only some of them. The possibilities for four chromosomes are shown in the diagram. to CO @&® a Letarv Plate O a7 @ 0 © v v at eR ce en Ss eC Nell ae oe eo Cree. 2 oa| ft 10 Filan, Spiimle 7 ge © Og? Oe Sy | y Bae “aN feat Sow Sats SS SAS & O ; Fia. 93. — Chromosomal cycle of P. caryecaulis. 182 HEREDITY AND SEX eggs and others small eggs? There must be, in all prob- ability, two kinds of parthenogenetic eggs produced by the stem mother—or at least there must be two kinds after the single polar body has been extruded.' In another group of animals, the daphnians, parthen- ogenetic species occur, that, in certains respects, are like the phylloxerans ; but these species illustrate also another relation of general interest. The fertilized winter egg produces always a female, the stem mother, which gives rise by parthenogenesis to offspring like herself, and the process may continue a long time. Each female produces one brood, then another and another. The last broods fail to develop, and this is a sign that the female has nearly reached the end of her life. But a parthenogenetic female may produce one or two large resting eggs instead of parthenogenetic females, and the same female may at another time produce a brood of males. The large resting eggs are inclosed in a thick outer protecting case. They must be fer- tilized in order to develop, yet they do not develop at once, but pass through an enforced, or a resting, stage that may be shortened, if the egg is dried and then returned to water. 1'The explanation may be found in the occurrence of two types of males — one type with two sex chromosomes, the other with one — two such types were actually figured in my paper. From the type with two sex chromosomes a stem mother would be produced with four sex chromosomes (two coming from the sexual egg). She would give rise to migrants with large eggs. From the type with one sex chromosome a stem mother would arise that produced small eggs with three sexchromosomes. According to whether two or one went out into the polar bodies of the small eggs, the two types of male would be reproduced. PARTHENOGENESIS 183 @ Veer QD Untisted female @ Wale Fic. 94. — Life cycle of Simocephalus; successive broods in horizontal lines, successive generations in vertical lines. (After Papanicolau.) 184 HEREDITY AND SEX In this life history we do not know what changes take place in the chromosomes. It has, however, often been claimed in this case that the transition from par- thenogenesis to sexual reproduction is due to changes in the environment. In fact, this is one of the stock cases cited in the older literature to show that sex is determined by external agents. It was said, that if the environment causes males to appear, then sex is determined by the environ- ment. But as a matter of fact, in so far as changes in the environment affect this animal, they cause it to cease reproducing by parthenogenesis, and induce sexual reproduction instead. The evidence is consistent in showing that any external change that affects the mode of reproduction at all calls forth either sexual eggs or males. The machinery of parthenogenesis is switched off, and that for sexual reproduction is turned on. The discrepancies that appear in the older accounts are probably due, as Papanicolau has shown, to dif- ferent observers using females that belong to different phases of the parthenogenetic cycle. Papanicolau, starting in each case with a winter egg, finds that as successive broods are produced the color of the par- thenogenetic eggs can be seen to undergo a progressive change from blue to violet. As the change progresses the chance that males and sexual eggs (‘‘females’’) will appear is greater. Until towards the end of the life of the individual the males and females come, as it were, of themselves (Fig. 94). If, however, individuals of successive broods are subjected to cold, it is found that while earlier broods do not respond, later ones respond PARTHENOGENESIS 185 more and more easily and change over to the sexual phase of the cycle. What has just been said about the successive broods might be said equally of the first-born offspring of the successive generations, as Papanicolau’s table shows (Fig. 94). Later born offspring respond more readily than do those that are historically nearer to the fer- tilized egg. It seems to me that these results become a little less obscure if we suppose some substance is produced during fertilization, that is carried by successive broods and successive descendants in an ever decreasing amount. As it becomes used up, the change is indicated by the color change in the egg. When it disappears, the sexual phase comes on. Its disappearance may be hastened by cold or by starvation. A third type, Hydatina senta (Fig. 95), an almost microscopic worm-like animal belonging to the rotifers, reproduces by parthenogenesis. The resting egg always gives rise to a parthenogenetic female, which also reproduces by parthenogenesis. Whitney has obtained 500 generations produced in this way. But from time to time another kind of individual appears. She is externally like the parthenogenetic female, but has entirely different capacities. Her eggs may be fertilized, and if they are they become resting eggs inclosed in a hard case. ‘The sperm enters when the eggs are immature and still in the ovary of the mother. The presence of a spermatozo6n in an egg determines that the egg goes on to enlarge and to pro- duce its thick coat. But if perchance no males are there to fertilize the eggs, this same female produces a 186 HEREDITY AND SEX crop of male eggs that develop into males without being fertilized at all. There are several facts of unusual interest in the HYDATINA SENTA Fevrlthen CGCHIALE (4) ay, coal ) Pa R Thale - egg pre Lcer. Fia. 95. — Life cycle of Hydatina senta. life history of hydatina, but we have occasion to consider only one of them. It has been claimed in this case also that external conditions determine the production of males, A more striking example of the erroneous- PARTHENOGENESIS 187 ness of this general conclusion would be hard to find ; for, in the first place, as we have seen, the same indi- vidual that produces males will produce out of the same eges females if she happens to be fertilized. In the second place the older evidence which was supposed to establish the view that certain specified changes in the environment cause the production of males has been overthrown. The French zodlogist, Maupas, is deserving of high praise for working out some of the most essential facts in the life cycle of hydatina, and for opening up a new field of investigation. But the evidence which he brought forward to show that by a low tempera- ture a high production of males is caused has not been confirmed by very careful and extensive repeti- tion of his experiments by Whitney and by A. F. Shull. The evidence that Nussbaum obtained which seemed to him to show that food conditions de- termined the production of males has likewise not borne the test of more recent work by Punnett, Shull, and Whitney. It has been found, however, that the production of the sexual phase of the cycle can be suppressed so that the animals continue almost indefinitely propagating by parthenogenesis. In several ways this may be accomplished. If hydatina is kept in a concentrated solution of the food culture, the sexual phase does not appear. The result has nothing to do with the abun- dance of food, for, if the food be filtered out from the fluid medium, the filtrate gives the same result. The following table given by Shull shows this very clearly. 188 HEREDITY AND SEX OLD CULTURE FILTRATE SprRiING WATER One-fourth One-half Three-fourths Undiluted 99139 | 999 | a eeiaame | | 350 | 8 | 362 | 0 | 337 %of oe! 12.8 5.7 4.1 2.1 0.0 Showing the number of male- and female-producers in the progeny of five sister individuals of Hydatina senta, one line being reared in spring water, the others in various concentrations of the filtrate from old food cultures. The extent of dilution of the medium is seen to be directly in proportion to the number of sexual forms that appear. If the solution be dried and the dry substance added to ordinary water, the same end is attained. It has not been possible to reverse the process and produce more sexual forms than are produced under ordinary conditions. This seems to mean that achange may be effected in one direction and not in the other. We cannot make a locomotive go faster than its mech- anism permits, with the most favorable conditions of fuel, oil, roadbed, and engineer; but if we put in stones in place of coal, we can bring it to a standstill. ARTIFICIAL PARTHENOGENESIS We have now considered some of the most striking examples of natural parthenogenesis in the animal kingdom. The facts show that fertilization of the egg is not im itself essential for development. The in- ARTIFICIAL PARTHENOGENESIS 189 dividuals that develop from parthenogenetic eggs are as vigorous as those from eggs that have been fertilized. We have seen that such eggs without being fertilized are capable of producing sexual females and males. In one case, at least, we have seen how the process is accomplished. When we review the facts of natural parthenogenesis, we find certain relations that arrest our attention. Most parthenogenetic eggs give off only a single polar body, while fertilized eggs without exception give off two polar bodies’ This difference is clearly con- nected with the fact that in parthenogenetic eggs the full number or diploid number of chromosomes is re- tained by the egg.! In fertilized eggs half the chromo- somes are thrown out in one of the two polar bodies. The number is made good by the chromosomes brought in by the spermatozoon. But this difference does not in the least explain nat- ural parthenogenesis; for we have experimental evi- dence to show, that an egg will develop when only half the number of chromosomes is present — one set will suffice. There is another fact about parthenogenetic eggs that has, I believe, been generally overlooked. Many of these eggs begin to develop into an embryo before they reach the full size of the fertilized eggs of the same species. This is true at least of the eggs of aphids, phylloxerans, daphnians, and rotifers. I interpret this 1 According to my observations on aphids and phylloxerans, the synapsis stage is omitted in parthenogenetic eggs, hence there is no union (or reduction) of the chromosomes. The omission of this stage may have something to do with parthenogenesis, although it is not evident what the relation may be. 190 HEREDITY AND SEX to mean that the eggs begin their development be- fore there has been produced over their surface a layer that in the mature egg seems to have an im- portant influence in restraining sexual eggs from de- velopment. | This brings us at once to a consideration of what keeps sexual eggs from developing until they are fer- tilized. In recent years a great variety of methods has been discovered by means of which sexual eggs can be made to develop without fertilization. This process is called artificial parthenogenesis. We owe especially to Professor Jacques Loeb the most successful accom- plishment of this important feat. The discovery in his hands has led to very great advances in our understanding of the developmental process. The chief importance of Loeb’s work les, in my opinion, not only in the production of embryos with- out fertilization (nature has long been conversant with such methods), but in other directions as well. First, it has thrown light on the nature of the in- hibitory process that holds back the sexual egg from developing until the sperm enters. Second, the information gained in this way tells us something of how the sperm itself may act on the egg and start it on its course. Third, it opens up the opportunity of studying cer- tain problems connected with the determination of sex that can be gained in no other way. Let me attempt briefly to elaborate some of these points. In many eggs, perhaps in all, a membrane is produced i. . . ae r ha ARTIFICIAL PARTHENOGENESIS 191 at the surface of the egg immediately after the sperm has entered. Here we have ocular evidence that fertilization effects a change in the surface layer of the egg. It has been shown that after this membrane is formed, the permeability of the egg to salts and other agents is affected and that the processes of oxidation are greatly accelerated. | In other words, the interior of the unfertilized egg is separated by means of its membrane from many things ‘in the surrounding medium — oxygen and the salts in sea-water, for example. The egg after fertilization lives in a new world. These same changes are brought about by those external agents that cause artificial parthenogenesis. But what an array of substances can cause the effect ! Many kinds of salts and of drugs, acids and alkalis, heat or cold, shaking or even sticking the surface of the egg with a minute needle. Loeb has shown that development depends not only on a change in the surface of the egg, but on other changes also. Hence his most successful methods are those in which two agents are applied successively to the egg — one affects primarily the surface, the other the interior of the egg. If, for example, the eggs are placed in a solution of a fatty acid, the membrane is produced. The egg is then removed to pure sea water from which oxygen has been driven out and left there for three hours. After its return to sea water it will produce a normal embryo. If, instead of putting the egg into water without oxygen, a hypertonic solution of salts is used (50 ce. 192 HEREDITY AND SEX of sea water plus 8 cc. of 2144 NaCl), the development may be carried through. Loeb concludes that the oxidations set up in the egg by a change in its outer surface affect the egg itself injuriously; and unless they are removed or the effects are counterbalanced by some other change (as when a hypertonic solution is used) the egg goes to pieces. Hence he believes that the sperm has a double réle in fertilization. First it changes the surface layer and increases in consequence the oxidations in the egg; second, the sperm brings into the egg some substance that counteracts poison produced by the oxidation itself. This is what fertilization accomplishes from a phystological point of view. In addition, we have seen that fertilization brings into the egg certain ma- terials whose presence affects the characters of the individuals that develop from it. This is what fertili- zation does from the point of view of the student of heredity. Let us turn for a moment, in conclusion, to the question of sex of anvmals that come from artificially parthenogenetic eggs. In natural parthenogenesis such eggs may de- velop into males, sexual females, or parthenogenetic females. But in artificial parthenogenesis the egg has already undergone reduction in its chromosomes and is repre- sented by half of the female formula as far as the chromosomes are concerned. The half formula will be XABC for the type with homozygous female. Since the egg has one X it may be expected to become ARTIFICIAL PARTHENOGENESIS 193 a male, but if sex is a relation of X to ABC, one cannot . be certain that it might not be a female. In cases where the female is heterozygous for the sex factor, as in birds and some sea urchins, the formula for the female would be XABCD — YABCD and for the male YABCD— YABCD. There would be two types of eggs, XABCD and YABCD. The former might be expected to produce a female, the latter prob- ably a male if such eggs were incited artificially to develop. Concerning the sex of the embryos so far produced by artificial parthenogenesis, we know of only two cases. These two cases are Delages’ result for the sea urchin, in which he got one male, and Loeb’s and Bancroft’s case for the frog, in which they believe that the two young obtained were females. What to expect on theoretical grounds is uncertain. We have only two facts that bear on the question. In the parthenogenetic eggs of the aphid, with the for- mula XABC ABC we get amale. In the case of the bee the formula is X ABC, which also givesa male. All else is hypothetical and premature, but if these two formule are correct, it appears that one X gives a male and that maleness is not due to a quantitative relation between X and one or two sets of the other chromosomes. It is the quantity of something in X, not the relation of this to the rest of the chromosomes. CHAPTER VII FERTILITY Darwin’s splendid work on cross- and self-fertiliza- tion, his study of the mechanism of cross-fertilization in orchids, and his work on the different forms of flowers of plants of the same species, mark the beginning of the modern study of the problem of fertility and sterility. Darwin carried out studies on -the effects of cross-fertilization in comparison with self-fertilization and reached the conclusion that the offspring resulting from cross-fertilization are more vigorous than the offspring from self-fertilization. No one can read his books dealing with these questions without being impressed by the keenness of his analysis and the open-minded and candid spirit with which the prob- lems were handled. Since Darwin’s time we lave not advanced very far beyond the stage to which Darwin carried these questions. We have more extensive experiments and some more definite ways of stating the results, but Darwin’s work still stands as the most important contribution that has been made to this subject. The credit of the second advance belongs to Weis- mann. His speculations concerning the effects of mixing of the germ-plasms of the two individuals, that combine at the time of fertilization, not only aroused renewed interest in the nature of the process of sexual reproduction, but brought to light also the 194 FERTILITY 195 effects of recombination of the different sorts of qualities contained in the parental strains. His attack on the hypothesis of rejuvenation that was so generally held at that time did very great service in exposing the mystical nature of such an imagined effect of cross- fertilization. In particular, Weismann’s endeavor to connect the theory of recombination with the facts of maturation of the egg and sperm has opened our eyes to possibilities that had never been realized before. His work has led directly to the third advance that has been made in very recent years, when the results of Mendelian segregation have been applied directly to the study of fertility and sterility. As I have said, Darwin’s work showed that cross- fertilization is generally beneficial. The converse proposition has long been held that continued inbreed- ing leads to degeneration and to sterility. This opinion rests largely on the statements of breeders of domesti- cated animals and plants, but there is also a small amount of accurate data that seems to support this view. I propose first to examine this question, and then consider what cross-fertilization is supposed to do, in the light of the most recent work. Weismann inbred white mice for 29 generations, and Ritzema-Bos bred rats for 30 generations. In each case the number of young per litter decreased in successive generations, more individuals were sterile and many individuals became weakened. This evi- dence falls in line with the general opinion of breeders. On the other hand, we have Castle’s evidence on inbreeding the fruit fly through 59 generations. He found some evidence of the occurrence of sterile pairs 196 HEREDITY AND SEX (mainly females), but we must be careful to distinguish between the appearance of sterile individuals in these cultures and the lessened fertility that may be shown by the stock in general. The recent work of Hyde on these same flies has shown that the appearance of sterile individuals may be an entirely different question from that of a decrease in general fertility. The latter again may be due to a number of quite different conditions. Castle and his co-workers found that the sterile individuals could be eliminated if in each genera- tion the offspring were selected from pairs that had not produced sterile individuals. Hyde has found, in fact, that one kind at least of sterile females owe their sterility to a definitely inherited factor that can be eliminated as can any other Mendelian recessive | trait. Moenkhaus, who has also extensively studied the problem of inbreeding in these flies has likewise found that his strains could be maintained at their normal rate of propagation by selecting from the more fertile pairs. If we eliminate from the discussion the occurrence of sterile individuals, the question still remains whether the output of the fertile pairs decreases if inbreeding is carried on through successive generations. There is some substantial evidence to show that this really takes place, as the following figures taken from Hyde’s results show. Py By Fs Ry. Bs Oi 368 209 “191° 184 - 65 119 156 At the end of thirteen generations the fertility of the stock was reduced by half, as determined in this FERTILITY 197 ease by the average number of flies per pair that hatch. But this is not a measure of the number of eggs laid or of those that are fertilized. , Whether inbreeding where separate sexes exist 1s sim- ilar to self-fertilization in hermaphroditic forms is not known. Darwin gives results of self-fertilization in [po- mea purpurea for ten generations. The effects vary so much in successive generations that it is not possible to state whether or not the plant has become less fertile. His evidence shows, however, that the cross- fertilized plants in each of the same ten generations are more vigorous than the self-fertilized plants, but this does not prove that the latter deteriorated. The problem has been studied in other ways. Some animals and plants propagate extensively by partheno- genesis; others by means of simple division. Whitney and A. F. Shull kept parthenogenetic strains of Hydatina senta for many generations. Whitney carried a strain of this sort through 500 generations. Towards the end the individuals became weak, the reproductive power was greatly diminished, and finally the strain died out. No attempt was made to breed from the more fertile individuals, although to some extent this probably occurred at times. If we admit that weakened individuals: appear sometimes in these lines and their weakness is inherited, then each time such an individual happened to be picked out a step downward would be taken; when the more fertile individuals chanced to be selected, the strain would be temporarily held at that level. But on the whole the process would be downwards if such downward changes are more likely to occur than upward ones. 198 HEREDITY AND SEX This is an assumption, but perhaps not an unreasonable one. Let me illustrate why I think it is not unreason- able. If the highest possible point of productivity is a complex condition due to a large number of things ‘that must be present, then any change is more likely to be downward, since at the beginning the high-water mark had been reached. In time casual selection would be likely to pick out a poor combination —if this hap- pened once the likelihood of return would be small. As we have seen (Chapter I) Maupas found in a number of protozoa that if he picked out an individual (after each two divisions) to become the progenitor of the next generation, the rate of division after a time slowed down. ‘The individuals became weaker and finally the race died out. Calkins repeated the experiments with paramcecium on a larger scale and obtained similar results. The question arose whether the results were not due to the hay infusion lacking certain chemical substances that in time produced an injurious effect. Calkins tested this by transferring his weakened strains to different culture media. The result was that the race was restored to more than its original vigor. But very soon degeneration again set in. A new medium again restored vigor to some degree, but only for a short time, and finally the oldest culture died out in the 742d generation. It was evident, therefore, that if the slackened rate of division and other evidences of degeneration were in part due to the medium, yet some of the effects produced were permanent and could not be effaced by a return to a more normal medium. Then came Woodruff’s experiments. He kept his paramececia on pith Rie Y 199 a mixed diet — on the kind of materials that it would be likely to meet with in nature, alternating with hay and other infusions. He found no degeneration, and at his last report his still vigorous strain was in’ the 3000th generation. How can we harmonize these different results? It is hazardous, perhaps, to offer even suggestions, but if we assume that in a medium not properly balanced paramoecium is likely to degenerate in the sense that it loses some of its hereditary factors, we can understand the failure to become normal when this has once taken place even in a new environment. ‘Temporarily the decrepit individual may be benefited by a change, but not per- manently if its hereditary mechanism is affected. In Woodruff’s experiment the normal environment brings about no degenerative changes in the hereditary mech- anism and the race continues to propagate indefinitely. Let us turn now to the other side of the question and see what results cross-fertilization has given. Hyde has found that if two strains of flies with low fertility are crossed, there 1s a sudden increase in the output, as seen in the diagram (Fig. 96). The facts show clearly an improvement. More eggs of each strain are fertilized by sperm from the other strain than when the eggs are fertilized by sperm from the same strain.! In this case the results are not due to a more fertile individual being produced (although this may be true) but to foreign sperm, acting better than the strain’s own sperm. The evidence, as such, does not show whether this is due to each strain having degenerated in certain directions, or to some other kind of a change in the heredity complex. 200 HEREDITY AND SEX The egg counts show that in the inbred stock many of the eggs are not fertilized, or if fertilized (82%) they still fail to develop. This means a decrease in fertility in the sense in which that word is here used. The offspring that arise from the cross-fer- tilization of these strains are more vigorous than their parents, if their increased fertility be taken as the measure of their vigor. The latter result is not shown in the table, for here 52% and 58% are the percent- ages of fertile eggs produced when the two strains are crossed. _Hislory of Inbred Sfock._ Fi. "2 | 3 4) NS Ge 7 6-8. 9" HO ieee 368. 209 19) 184 @ #9 - -—- - = = = 156 Cross of Fiz by Truncale Truncated by Truncale d 24% Fi3s =FygQ 32% 5R% 58% Fia. 96. — The horizontal line F:—-F\3 gives the average number of flies per pair that emerged from inbred stock, decreasing from 368 to 156 per pair. Below is shown the results of a cross between a race of Truncates (short wings) and Fis. The percentages here give the number of eggs that hatched in each case. Darwin found that cross-fertilization was bene- ficial in 57 species of plants that he studied. In the 1'The upper line F;—F; gives the average output of flies per pair. Below this line the percentages mean the number of isolated eggs that hatched. FERTILITY 201 case of primula, which is dimorphic, he found not only that self-fertilization gave less vigorous plants, but that when pollen from a long-styled flower of one plant fertilizes the pistil of another long-styled plant the vigor of the offspring is less than when the same kind of pollen is used to fertilize the pistil of a short-styled flower. The next table gives the detailed results. Maximum | Minrimum | AVERAGE NUMBER OF | NUMBER % OF SEEDSIN|OF SEEDSIN| NO. OF NATURE OF UNION FLOWERS OF SEED Any ONE | ANy ONE | SEEDS PER FERTILIZED | CAPSULES CAPSULE CAPSULE CAPSULE Long-styled form by pollen of — short- 10 6 62 34 46.5 styled form: Legitimate union. Long-styled form by own-form pollen: 20 4 49 2 20.0 Illegitimate union. Short-styled form by Be eg 10 8 61 37 47.7 styled form: Legitimate union. Short-styled form by own-form pollen: 17 3 19 9 12.1 Illegitimate union. The _two legitimate 20 14 62 34 47.1 unions together. The two illegitimate 37 - 49 9 21.0 unions together. We know now that these two types of plants — long- styled and short-styled — differ from each other by a single Mendelian factor. We may therefore state 202 HEREDITY AND SEX Darwin’s result in more general terms. The hetero- zygous plant is more vigorous than the homozygous plant. Moreover, in this case it is not the presence of the dominant factors that makes greater vigor (for the short-styled plant containing both dominants is less vigorous than the heterozygous), but the presence of two different factors that gives the result. At left of figures there are two strains of pure bred corn and HIG. 97. at right the hybrids produced by crossing those two pure strains. (After East.) The most thoroughly worked out case of the effects of inbreeding and cross-breeding is that of Indian corn. In recent years East and G. H. Shull have studied on a very large scale and with extreme care the problem in this plant. Their results are entirely in accord on all essential points, and agree with those of Collins, who has also worked with corn. Kast and Shull find that when two strains of corn vy sid oh oO 6 ey Mal ba 203 (that have been to a large extent made pure) are crossed, the offspring is more vigorous than either:parent (Fig. Fig. 98. — At left an ear of Leaming Dent corn, and another at right after four years of inbreeding. The hybrid between the two is shown in the middle ear. (After East.) 97). This is clearly shown in the accompanying pie- tures. Not only is the hybrid plant taller and stronger, but in consequence of this, no doubt, the yield of corn 204 HEREDITY AND SEX per bushel is much increased, as shown in the next figure (Fig. 98). When the vigorous F; corn 1s self-fertilized, it produces a very mixed progeny, more variable than itself. Some of the F, offspring are like the original grandparental strains, some like the corn of first generation, and others are intermediate (Fig. 99). 4 No. Noid, CAXD F, ei, 7 bu peracre 46.6 bu perocre : HAS bu. peracre: Fie. 99. — No. 9 and No. 12, two inbred strains of Leaming Dent corn compared with F; and F2 (to right). (After East.) It will not be possible for us to go into an analysis of this case, but Shull and East have shown that the results are in full harmony with Mendelian principles of segregation. The vigor of the fF; corn is explained on the basis that it is a hybrid product. To the extent to which the two parent strains differ from each other, so much the greater will be the vigor of the offspring. This seems an extraordinary conclusion, yet when tested it bears the analysis extremely well. Shull and apparently East also incline to adopt the FERTILITY 205 view that hybridity or heterozygosity itself is the basis for the observed vigor; but they admit that another interpretation is also possible. For instance, each of the original strains may have been deficient in some of the factors that go to make vigor. Together they give a more vigorous individual than themselves. Whitney ran one line of hydatina through 384 par- thenogenetic generations, when it died (Line A). An- other line was carried through 503 generations, and at the last report was in a very weakened condition (Line B). When the former line was becoming extinct, he tried inbreeding. From the fertilized eggs he ob- tained a new parthenogenetic female. It showed scarcely any improvement. The other line gave similar results. In one case he again inbred for a second time. He found that the rates of reproduction of lines A and B were scarcely, if at all, improved. Whitney then crossed lines A and B. At once an improvement was observed. The rate of reproduction (vigor) was as great as that in a control line (reared under the same conditions) that had not deteriorated. The experiments of A. F. Shull on hydatina were somewhat different. He began with the twelfth gen- eration from a sexual egg. The line was supposedly not in a weakened condition. He inbred the line and obtained from the fertilized egg a new parthenogenetic series. After a few generations he inbred again. The results are shown in the next table. It is clear that there has been a steady decline despite sexual repro- duction, measured by four of the five standards that Shull applied, namely, size of family of parthenogenetic females, and of sexual females, number of eggs per day, 206 HEREDITY AND SEX SHOWING DECREASE OF V1IGOR, AS MEASURED BY VARIOUS CHAR- ACTERS, IN Stx SuccESSIVELY INBRED PARTHENOGENETIC LINES oF Hydatina senta NUMBER OF PARTHENOGENETIC nD LINE a CHARACTER TO BE MEASURED on a 1 2 3 4 5 6 I. | Size of family of parthenogenetic female . . |48.4 142.5 |46.8 142.5 131.0 |22.6 Size of family of fertilized sexual female . . (16.7 |12.8 |12.8 |11.5 | 6.3 | 7.3 Number of eggs laid perday . . . » 5 (L1.0.111.4- 110-3) 100 Number of days required to reach maturity | 2.27| 1.66) 2.25) 1.93 2.25] 2.12 Proportion of cases in which first daughter | aid not become parente + a) a.) sn een Lyd | 1/3 | 2/4 |3/16| 0/4 | 5/8 HamMerin Percentages! une, here Ce ee 14.2 25.0 41.6 II. | Size of family of parthenogenetic female . . |48.4 |30.8 |41.0 |37.0 |33.8 |24:8 Size of family of fertilized sexual female . . ,16.7 |13.7 |13.5 (15.2 LOE 177.6 Number of eggs laid perday .. . 11.07 | L1.6 FeO Fez eo OG Number of days required to reach mat Seat PPA Mays, pe 2.20} 1.90} 2.00 | Proportion of cases in which first daughter | did:notibecome:parent i) ose Se LY arg 2/7 | 2/10 8/20 | 7/16 Same in percentages 25.0 23. 5 41.6 number of times the first daughter was too weak to become the mother of a new line. It is clear that inbreeding did not lead to an increase in vigor. In paramcecium there is also some new evidence. Calkins in 1904 brought about the conjugation of two in- dividuals of a weak race in the 354th generation. From one of the conjugants a new line was obtained that went through another cycle of at least 376 generations in culture, while during the same time and under sim- ilar conditions the weakened race from which the con- jugants were derived underwent only 277 generations. Jennings has recently reported an experiment in which some paramoecia, intentionally weakened by breeding in a small amount of culture fluid, were FERTILITY 207 allowed to conjugate. Most of the lines that descended from several pairs showed no improvement but soon died out. In only one case was an individual produced that was benefited by the process. Jennings’ results are, however, peculiar in one very important respect. He did not use a race that had run down as a result of a long succession of generations, but a race that he had weakened by keeping under poor conditions. We do not know that the result in this case is the same as that in senile races or inbred races of other workers. It is not certain that the hereditary complex was affected in the way in which that complex is changed by inbreeding. He may have injured some other part of the mechanism. Jennings interprets conjugation in paramoecium to mean that a recombination of the hereditary factors ~ takes place. Some of these combinations may be more favorable for a given environment than are others. Since these will produce more offspring, they will soon become the predominant race. The next diagram (Fig. 100) will serve to recall the principal facts in regard to conjugation in paramce- cium. ‘Two individuals are represented by black and white circles. At the time of conjugation the small or micronucleus in each divides (B), each then divides again (C). Four nuclei are produced. One of these micronuclei, the one that les nearest the fusion point, divides once more, and one of the halves passes into the other individual and fuses there with another nucleus. The process is mutual. Separation of the two indi- viduals then takes place and two ex-conjugants are formed. Each has a new double nucleus. This nu- 208 HEREDITY AND SEX cleus divides (@) and each daughter nucleus divides again (H), so that each ex-conjugant has four nuclei. A = B = = G SBSFEee e& D Bp S E 7 = F = {7 oe is CS ee = = G = = = = = = asbet = : 5 = ye 6 6 EEC 6 eé Se & & 646 eé = PSCECCSCC?CTCSCST ! SCSCRECSESC ES z “a o gy GQ Pa Sy. GQ DS > € &€ €- E€E4€E€€E Fic. 100. — Diagram to show the history of the micronuclei of two Parameecia during (A-F) and after (F—J/) conjugation. Compare this dia- gram with Fig. 2. Another division gives eight nuclei in each. ‘The para- moecium itself next divides —each half gets four nuclei. A second division takes place, and each gets two of the nuclei. Four new individuals result. In each of FERTILITY 209 these individuals one of the nuclei remains small and becomes the new micronucleus, the other enlarges to form the new macronucleus. Thus from each ex- conjugant four new paramoecia are produced, which now proceed to divide in the ordinary way, 1v.e. the micronucleus and the macronucleus elongate and divide at each division of the animal. It is customary to regard some phase in this process as Involving a reduction division in the sense that a separation of the paired factors takes place. If this occurs prior to interchange of micronuclei (/), then each ex-conjugant corresponds to an egg after fertilization. It is conceivable, however, that segregation might oc- cur in the two divisions that follow conjugation, which would give a different. interpretation of the process than the one followed here. On the first of these two hypotheses two new strains result after conjugation. Each is a recombination of factors contained in the two parents. If the two par- ents were alike, 7.e. homozygous, in many factors, and different, 7.e. heterozygous, in a few, the two individuals would be more alike than were the original races from which they came. ‘This is, in fact, what Jennings has shown to be the case, at least he has shown that on the average the ex-conjugants are more like each other than were the original strains. Calkins has obtained some new and important facts concerning the likeness and unlikeness of the new strains that result from conjugation. He has used wild, z.e. not weakened, individuals, and has followed the history of the four lines resulting from the first four individuals produced by each ex-conjugant. The 210 HEREDITY AND SEX history of six such ex-conjugants is shown in the next diagram (Fig. 101). The four lines, ‘‘ quadrants,” (1, 2, 3, 4) that are descended from each of six ex- conjugants (viz. G, H, L, M, Q, B) are shown. At intervals large numbers of the populations were put under conditions favorable to conjugation and the Fayugalion variations tr Stix tx-coryuguntls of Sarumeeium caudadum Deée Dec Dee rs Pa Pairs Patrs DPD y | ape June tet Feb pest (nat rest Ca of ‘est # “ nie ters A at was ae pt st” = Q 4.9—46I0 if hos ™is O x | ia 3 G Serces} i Z #2113. rae O59 od (4ft3 IG = 914 5 =< 4 re 2 ©) Fig. 101. — History of six (G, H, L, M, Q, B) ex-conjugants. In each the descendants of the first four individuals (after conjugation) is shown; the numbers indicate the pairs of conjugants counted when the test was made. X indicates deaths; O indicates that no conjugation took place. (After Calkins.) number of conjugating pairs counted. The results are shown in the diagram. The circles indicate no conjugations; XY indicates the death of the strain. In the G and in the M series many conjugations took place. In other series conjugation did not take place until much later. Striking differences appear in the different quadrants although they were kept under similar conditions. FERTILITY 211 But even amongst the four lines descended from the same ex-conjugant marked differences exist. These differences cannot be attributed to constitutional dif- ferences unless a segregation of factors takes place after conjugation or unless it can be shown that these differences are not significant. In the light of these conflicting results on paramoecium it may seem unsafe to draw any far-reaching conclusions concerning the nature of sexual reproduction in general from the evi- dence derived from these forms. In the higher animals, however, the evidence that segregation takes place prior to fertilization and that recombinations result can scarcely be doubted. THEORIES OF FERTILITY Let us now try to sum up the evidence in regard to the influence of cross-fertilization. This can best be done by considering the three most important hypoth- eses that have been brought forward to explain how crossing gives greater vigor. Shull and East explain the vigor of the hybrid by the assumption that it contains a greater number of dif- ferent factors in its make-up than either of its parents. They support the view by an appeal to the next (/2) generation from such hybrids that shows a lower range of vigor, because, while a few individuals of this generation will be as mixed as the hybrid (/1), and therefore like it, most of them will be szmpler in com- position. This interpretation is also supported by the evidence that when pure lines (but not necessarily, however, homozygous lines) are obtained by self-fer- tilizing the offspring of successive generations from 212 HEREDITY AND SEX these first hybrids, further decline does not take place. An alternative view, that is also Mendelian, has been offered by Bruce and by Keeble and Pellew. Vigor, it is maintained, is in proportion to the number of domi- nant factors, and in proportion to the number of these factors present whether in a hybrid or in a homozygous (duplex) condition. On this view the hybrid is vigorous, not because it is hybridous, so to speak, but because in its formation a larger number of dominant factors (than were pres- ent in either parent) have been brought together. A third view is also compatible with the evidence, namely, that there may exist factors that are them- selves directly concerned with fertility. There is one such case at least that has been thoroughly analyzed by Pearl. Pearl studied for five years the problem of fertility in two races of fowls, viz. barred Plymouth rocks and Cornish Indian games. The main features of his results are shown in the diagram (Fig. 102). He finds that the winter output of eggs, which is correlated with the total production, is connected with two factors. One factor, designated by L, is a non-sex-linked char- acter. Ifit is present, an average of less than 30 eggs is produced in the winter season. There is another factor, L2, that is present in the barred rocks, but not — in the Indian game. If present alone, the winter out- put is again about 30 eggs on an average. If, how- ever, both LZ, and ZL, are present, the winter output is more than 30 and may be as great as 90, or in rare cases 100-120 eggs. FERTILITY 213 The peculiarity about this discovery is that the second factor, Lz, is sex-linked, which means in this case that it is carried by the eggs that will produce the males in the next generation, and not by the eggs that will produce the daughters. Hence if the daughters of high- producing hens are selected, one does not get in them Inherilance of Ferfilify in Fowl. (Pearl) Low? fF.L,—L, Flea—Leg Low? (Zero od) 4,— Ly Lg— lz Zerd FLO Low File te Zero? Lilt (Low) hele (Low)d hats oerreaaad Lot, (Low) J FL, 4abet, High L, tz het, High)c 3 ta ty (Low) o i, Q Fle how fe) eae Uh (High) o pate Fi Ne s ff Le— te FL, High? tah, EHighd eet Obie, to (Lows Fig. 102. — Illustrating Pearl’s hypothesis. F = female factor present in half of the eggs and determining sex. JL, = factor for low egg produc- tion; i, its allelomorph for zero production of winter eggs. IL» = factor for high winter production; /:, its allelomorph. the high productiveness of the mother. It is her sons that inherit the character, although they cannot show it except in their offspring. Aside from whatever practical interest these results may have, the facts are important in showing that such a thing as a factor for fertility itself may be present, without otherwise being apparent, and that this factor 214 HEREDITY AND SEX taken in connection with another (or others) gives high productivity. The other point to which I wish to call attention relates to a different matter. We have met with some eases where lowered fertility was due to eggs failing \ Fig. 103. — Normal male of Drosophila (on left) and male with ‘‘rudi- mentary’’ wings (on right). Note sex comb (lower left). to a greater or less degree to be fertilized by sperm of the same strain. A striking case of this kind is found in a mutant of the fruit fly that appeared in my cultures. The mu- tant has rudimentary wings (Fig. 103). The females are absolutely infertile with males of the same kind. FERTILITY 215 If they are mated to any other male of a different strain, they are fertilized. The males, too, are capable of fer- tilizing the eggs of other strains, in fact, are quite fertile. The factor that makes the rudimentary winged fly is of such a sort that it carries infertility along with it —in the sense of self-infertility. This result has nothing to do with inbreeding, and the stigma cannot be removed by crossing out and extracting. A somewhat similar factor, though less marked, is found by Hyde in certain of his inbred stock to which I have referred. As his experiments show, the infer- tility in this case is not due to lack of eggs or sperm, but to a sort of incompatibility between them so that not more than 20 per cent of the eggs can be fertilized by males of the same strain. In the flowering plants where the two sexes are often combined in the same individual, it has long been known that there are cases in which self-fertilization will not take place. The pollen of a flower of this kind if placed on the stigma of the same flower or of any other flower on the same plant will not fertilize the ovules. Yet the pollen will fertilize other plants and the ovules may be fertilized by foreign pollen. Correns has recently studied that problem and has arrived at some important conclusions. He worked with a common plant, Cardamine pretensis. In this plant self-fertilization is ineffectual. He crossed plant B with plant G, and reared their offspring. He tested these with each other and also crossed each of them back to its parents that had been kept alive for this pur- pose. The latter experiment is simple and more in- 216 HEREDITY AND SEX structive. His results and his theory can best be given together. Correns assumes that each plant contains some factor that produces a secretion on the stigma of the flowers. This secretion inhibits the pollen of the same plant from extending its pollen tube. He found, in fact, that the pollen grains do not grow when placed on the stigma of the same plant. All plants will be hybril Back Cross tn Cardamine pidindst (Conc) OE Boat Cane Gang BG oe or B — 3 Fg gp Be hg ) + at Fig. 104. — Illustrating the crossing of the types Bb and Gg to give four classes: BG, Bg, bG, bg. Each of these is then back-crossed either to B or to G with the positive (+) or negative (—) results indicated in the diagram. for these factors, hence plant B will produce two kinds of germ-cells, B and b. Similarly, plant G will produce two kinds of germ-cells, G-g. If these two plants are crossed, four types will be produced. When these are back-crossed to the parents, the expectation is shown in the diagram (Fig. 104). Half the combination should be sterile and half should be fertile. This is, in fact, what occurs, as shown in the same diagram. ‘The — signs indicate that fertilization does not occur, while the + signs indicate successful fertilization. Correns’ theory is also in accord with other com- FERTILITY 217 binations that he made. There can be little doubt that he has pointed out the direction in which a solu- tion is to be found. There is a somewhat similar case in animals. In one of the Ascidians, Ciona intestinalis, an hermaphrodite, the sperm will not fertilize the eggs of the same indi- vidual. But the sperm will fertilize eggs of other individuals, and vice versa. Castle first found out this fact, and I have studied it on a large scale. The diagram (Fig. 105) gives an example of one such ex- periment made recently by W. 8. Adkins. Five individuals are here used. The eggs of one. individual, A, were placed in five dishes (horizontal line); likewise those of 6, C, D, EH. The sperm of A, designated by a (vertical lines) was used to fertilize the eggs, A, B, C, D, EH; likewise the sperm 0, c, d, e. The self-fertilized sets form the diagonal line in the diagram and show no fertilization. The other sets show various degrees of success, as indicated by the percentage figures. These results can best be under- stood, I think, by means of the following hypoth- esis. The failure to self-fertilize, which is the main problem, would seem to be due to the similarity in the hereditary factors carried by eggs and sperm; but in the sperm, at least, reduction division has taken place prior to fertilization, and therefore unless each animal was homozygous (which from the nature of the case cannot be assumed possible) the failure to fertilize cannot be due to homozygosity. But both sperm and eggs have developed under the influence of the total or duplex number of hereditary factors; hence they are alike, 2.e. their protoplasmic substance has been 218 HEREDITY AND SEX under the same influences. In this sense, the case is like that of stock that has long been inbred, and has SUE and Cross Fertilization i Gore. Fic. 105. — The oblique line of letters A%, B®, C°, D4, E®, gives the self- fertilized sets of eggs; the rest A?, A°, etc., the cross-fertilized sets. A, B, C, D, E = eggs; a, b, c, d, e, = sperm of same individuals. (From unpub- lished work of W. S. Adkins.) come to have nearly the same hereditary complex. if this similarity decreases the chances of combination be- tween sperm and eggs, we can interpret the results. Cor- rens’ results may come under the same interpretation. FERTILITY 219 I have tried to bring together the modern evidence that bears on the problems of fertility and sterility. It is evident that there are many obscure relations that need to be explained. I fear that, owing to the diffi- culty of summarizing this scattered and diverse ma- terial, I have failed to make evident how much labor and thought and patience has been expended in ob- taining these results, meager though they may appear. But while it is going to take a long time and many heads and hands to work out fully these problems, there can be little doubt that the modern method is the only one by which we can hope to reach a safe conclusion. CHAPTER VIII SPECIAL CASES OF SEX-INHERITANCE THE mechanism of sex-determination that we have examined gives equal numbers of males and females. But there are known certain special cases where equality does not hold. I have selected six such cases for discussion. Each of these illustrates how the mechan- ism of sev-determination has changed to give a different result ; or how, the mechanism remaining the same, some outside condition has come in that affects the sex ratio. It is so important at the outset to clearly recognize the distinction between sex-determination and _ sex ratio, that I shall take this opportunity to try to make clear the meaning of this distinction. The failure to recognize the distinction has been an unfailing source of misunderstanding in the literature of sex. (1) A hive of bees consists of a queen, thousands of workers, and at certain seasons a few hundred drones or males. The workers are potentially females, and these with the queen give an enormous preponderance of females. In this case the explanation of the sex ratio is clear. Most of the eggs laid by the queen are fertilized, and in the bee all fertilized eggs become fe- males, because as we have seen there is only one class of spermatozoa produced, and not two as in other insects. There is a parallel and interesting case in one of the wasps described by Fabre. The female lays her eggs 220 SPECIAL CASES OF SEX-INHERITANCE 221 as a rule in the hollow stems of plants, each egg in a separate compartment. In some of the compartments she stores away much more food than in others. From these compartments large females hatch. From com- partments where less food is stored the smaller males are produced. It may seem that the amount of food stored up determines the sex of the bee. To test this Fabre took out the excess of food from the large compartments. The wasp that emerged, although small for want of food, was in every case a female. Fabre enlarged the smaller compartments and added food. The wasp that came out was a male, larger than the normal male. It is evident that food does not determine the sex, but the mother wasp must fertilize the eggs that she lays in chambers where she has stored up more food, and not fertilize those eggs that she deposits in com- partments where she has accumulated less food. (2) A curious sex ratio appeared in one race of fruit flies. Some of the females persisted in producing twice as many females as males. This was first discovered by Miss Rawls. In order to study what was taking place, I bred one of these females that had red eyes to a white-eyed male of another stock. All the offspring had red eyes, as was to be expected. I then bred these daughters individually to white-eyed males again (Fig. 106). Half of the daughters gave a normal ratio; the other half gave the following ratio: Red | Red White White 4 3 3 50 0 50 50 222 HEREDITY AND SEX It is evident that one class of males has failed to ap- pear — thered males. If we trace their history through these two generations, we find that the single sex chro- % ? fe) Fic. 106. — Diagram to show the heredity of the lethal factor (carried by black X). . fee eae eee ae Germany. ..: 40. o=1288 AUSELIO 9h a ee ee eee Hungary 2) see ee OULU Switzerland. . . . .:. 135.0 } to 100 females Belriuim es. 1 pee lo Hollands 33-2 ete eee Sweden? oy ox 5s eee eee oo IN OFWAY fn se ce eee eG Denmark yi eee reat ta And if abortive births are also taken into account, the ratio is still higher. It seems that the male embryo is not so strong as the female, or else less likely, from other causes, to be born alive. In many of the domesticated animals also, especially SPECIAL CASES OF SEX-INHERITANCE 231 the mammals, there is an excess of males at birth, as the next table shows. MaALrEs FEMALES frotseees. eee ke ik 98.31 100 (Diising) Pe etre ee oe O78 100 (Wilchens) SuCopE fee <.cPee a COTS. 100 (Irwin) Pigmeeeen ers. See L118 100 (Wilchens) Piette et ee LOO 100 (Cuénot) ereweete.. x. a J.e LOO:O 100 (Cuénot) Sema rant, ba as Pate OAL 7. 100 (Darwin) A little later I shall bring forward the evidence that makes probable the view that in man the mechanism for sex-determination is like that in other animals, where two classes of sperm are produced, male- and female-producing. How then can we account in the human race for the excess of eggs that are fertilized by male-producing spermatozoa? At present we do not know, but we can at least offer certain suggestions that seem plausible. In mammals the fertilization occurs in the upper parts of the oviduct. In order to reach these parts the sperm by their own activity must traverse a dis- tance relatively great for such small organisms. If the rate of travel is ever so slightly different for the two classes of sperm, a differential sex ratio will occur. Again, if from any cause, such as disease or alcoholism, one class of sperm is more affected than the other, a disturbance in the sex ratio would be expected. At present these are only conjectures, but I see no ground for seizing upon any disturbance of the ratio in order to formulate far-reaching conclusions in regard to sex-determination itself. As I pointed out in the beginning of this chapter, we may go 232 HEREDITY AND SEX wide of the mark if we attempt to draw conclusions ‘concerning the determination of sex itself from devia- tions such as these in the sex ratio, yet it is the mistake that has been made over and over again. We must look to other methods to give us sufficient evidence as to sex-determination. Fortunately we are now in a position to point to this other evidence with some assurance. With the mechanism itself worked out, we are in a better position to explain slight variations or variables that modify the combinations in this way or in that. THE ABANDONED VIEW THAT EXTERNAL CONDITIONS DETERMINE SEX But before taking up the evidence for sex-determina- tion in man I must briefly consider what I have been bold enough to eall the abandoned view that external conditions determine sex. Let us dismiss at once many of the guesses that have been made. Drelincourt recorded 262 such guesses, and Geddes and Thomson think that this number has since been doubled. Naturally we cannot consider them all, and must confine ourselves to a few that seem to have some basis in fact or experiment. The supposed influence of food has been utilized in a large number of theories. The early casual evidence | of lLandois, of Mrs. Treat, and of Gentry has been entirely set aside by the careful observations of Riley, Kellogg and Bell, and Cuénot. In the latter cases the experiments were carried through two or even three generations, and no evidence of any influence of nourishment was found. SPECIAL CASES OF SEX-INHERITANCE 2393 The influence of food in sex-determination in man has often been exploited. It is an ever recurrring episode in the ephemeral literature of every period. The most noted case is that of Schenk. In his first book he said starvation produced more females; in his second book he changed his view and supposed that starvation produces more males. | Perhaps the most fertile source from which this view springs is found in some of the earlier statistical works, especially that of Diising. Diising tried to show that more girls are born in the better-fed classes of the com- munity, in the poorer classes more boys. The effective difference between these two classes is supposedly one of food! For instance, he states that the birth-rate for the Swedish nobility is 98 boys to 100 girls, while in the Swedish clergy the birth-rate is 108.6 boys to 100 girls. Other statistics give exactly opposite results. Pun- nett found for London (1901) more girls born amongst the poor than the rich. So many elements enter into these data that it is doubtful if they have much value even in pointing out causes that affect the sex ratio, and it is quite certain that they throw no light on the causes that determine sex. In other mammals where a sex ratio not dissimilar to that in man exists, extensive experiments on feeding have absolutely failed to produce any influence on the ratio. We have, for instance, Cuénot’s experi- ments with rats, and Schultze’s experiments with mice. The conditions of feeding and starvation were much more extreme in some cases than is likely to occur ordinarily, yet the sex ratio remained the same. Why in the face of this clear evidence do we find 234 HEREDITY AND SEX zoOlogists, physicians, and laymen alike perpetually discovering some new relation between food and sex? It is hard to say. Only recently an Italian zoGlogist, Russo, put forward the view that by feeding animals on lecithin more females were produced. He claimed that he could actually detect the two kinds of eggs in the ovary — the female- and the male-producing. It has been shown that his data were selected and not complete; that repetition of his experiments gave no confirmative results, and probably that one of the two kinds of eggs that he distinguished were eggs about to degenerate and become absorbed. But the food theories will go on for many years to come — as long as credulity lasts. Temperature also has been appealed to as a sex fac- tor in one sense or another. R. Hertwig concluded that a lower temperature at the time of fertilization gave more male frogs, but -Miss King’s observations failed to confirm this. There is the earlier work of Maupas on hydatina and the more recent work of von Malsen on Dinophilus apatris. I have already pointed out that Maupas’ results have not been con- firmed by any of his successors. Even if they had been confirmed they would only have shown that tempera- ture might have an effect in bringing parthenogenesis to an end and instituting sexual reproduction in its stead. In hydatina the sexual female and the male producing individual are one and the same. A more striking case could not be found to show that the en- vironment does not determine sex but may at least change one method of reproduction into another. There remain von Malsen’s results for dinophilus, SPECIAL CASES OF SEX-INHERITANCE 235 where large and small eggs are produced by the same female (Fig. 109). The female lays her eggs in clus- ters, from three to six eggs, as a rule, in each cluster. The large eggs produce females; the small eggs pro- Fia. 109. — Dinophilus gyrociliatus. Females (above and to left) and males (below and to right). Two kinds of eggs shown in middle of lower row. (After Shearer.) duce rudimentary males that fertilize the young fe- males as soon as they hatch and before they have left the jelly capsule. Von Malsen kept the mother at different tempera- tures, with the results shown in the table. The ratio of small eggs to large eggs changes. But the result TEMPERATURE oars 3 2 aoe ante Room temp. 19°C. . 202 | 327 813 224 5,6 (ibis Oh ae. 925 973 2975 1: 3,5 4,2 Ine et 6 383 507 886 id Bye Bae a) a 50) 236 HEREDITY AND SEX obviously may only mean that more of the large eggs are likely to be laid at one temperature than at another. In fact, temperature seemed to act so promptly accord- ing to Von Malsen’s observations that it 1s very un- likely that it could have had any influence in deter- mining the kind of egg produced, but rather the kind of egg that was more likely to be laid. We may dis- miss this case also, I believe, as not showing that sex is determined by temperature. SEX-DETERMINATION IN MAN Let us now proceed to examine the evidence that bears on the determination of sex in man. I shall draw on three sources of evidence: 1. Double embryos and identical twins. 2. Sex-linked inheritance in man. 3. Direct observations on the chromosomes. The familiar case of the Siamese twins is an example of two individuals organically united. A large series of such dual forms is known to pathologists. There are hundreds of recorded cases. In all of these both individuals are of the same sex, 2.e. both are males or both are females. There is good evidence to show — that these double types have come from a single fer- tilized egg. They are united in various degrees (Fig. 110); only those that have a small connecting region are capable of living. These cases lead directly to the formation of separate individuals, the so-called identical twins. | Galton was one of the first, if not the first, to recognize that there are two kinds of twins — identical twins and ordinary or fraternal twins. SPECIAL CASES OF SEX-INHERITANCE 237 Identical twins are, as the name implies, extremely alike. They are always of the same sex. There is every presumption and some collateral evidence to show that they come from one egg after fer- tilization. On the other hand, amongst ordinary twins a boy and a girl, or two boys and two girls, occur in the ratio expected, 7.e. on the basis that their sex is RRR ROR BA RRA AEKE NM yor de 06 NH 6 eB \ Gs DIAGRAM SHOWING THE INTERRELATIONS OF THE VARIOUS SORTS OF DIPLOPAG! AND * ‘DUPLICATE TWINS, ILLUSTRATIVE OF THE THEORY. ADVANCED IN THIS PAPER. FURTHER EX= PLANATION IN THE TEXT. Fig. 110. — Diagram showing different types of union of double monster (After Wilder.) not determined by a common external or internal cause. Since fraternal twins and identical twins show these relations at birth and from the fact that they have been in both cases subjected to the same condi- tions, it follows with great probability that sex in such cases is determined before or at the time of fertilization. : This conclusion finds strong support from the condi- 238 HEREDITY AND SEX tions that have been made out in the armadillo. Jehring first reported that all the young of a single litter are of the same sex (Fig. 111). The statement has been verified by Newman and by Patterson on a large scale. In addition they have found, first, that only one egg leaves the ovary at each gestative period ; and second, that from the egg four embryos are pro- Fig. 111. — Nine-banded Armadillo. Four identical twins with a common placenta. (After Newman and Patterson.) duced (Fig. 112). The material out of which they develop separates from the rest of the embryonic tissue at a very early stage. The four embryos are identical quadruplets in the sense that they are more like each other than like the embryos of any other litter, or even more like each other than they are to their own mother. The second source of evidence concerning sex-deter- SPECIAL CASES OF SEX-INHERITANCE 239 mination in man is found in the heredity of sex-linked characters. The following cases may well serve to illustrate some of the better ascertained characters. The tables are taken from Davenport’s book on ‘“‘ Heredity in Relation to Eugenics.’ The squares indicate males, affected males are black squares ; the heavy circles indi- cate females, that are supposed to carry the factors, but Fig. 112. — Nine banded Armadillo. Embryonic blastocyst that has four embryos on it, two of which are seen in figure. (After Newman and Patterson.) such females do not exhibit the character themselves. Solid black circles stand for affected females. Hemophilia appears in affected stocks almost ex- clusively in males (Fig. 113). Such males, mating with normal females, give only normal offspring, but the daughters of such unions if they marry normal males will transmit the disease to half of their sons. Affected females can arise only when a hemophilious male marries a female carrying hemophilia. If we HEREDITY AND SEX 40) (}IOdueaeq WO.) “Wosso'T 0} Sulpioooe ‘Apres Jodureyy JO veiZpog = “elrydowyy yo AyIpossz]T — “ETT ‘pI a "er ae OOM OC) 7 O LI SPECIAL CASES OF SEX-INHERITANCE 241 = SS XO UM a/ se <)> Q Ss XX XO SD BKK XX XO XO Fria. 114. — Diagram to indicate heredity of color blindness through male. aN A) | a #9 N= Ke , / t j es*y ‘ } \ Bors | ” j st a / ag: bide 7 } ve Ne, ee -¢lk =k. Whit ee NEN , oe d of /; f : Ze whats ’ q) © a. 6, $ aie, wet tis Noghy ’ i gett fe ‘ (oN re ae ; ee ; h aie t fa, Soe Ae ea, @)> Al ar % a ett Poy} LJ fixes ae +3 “~ “9? ee t9° a2, 4 , Aas a kee . ie, eae J 2 k f re Fia. 120. — Human spermatogenesis according to von Winiwarter. a, spermatogonial cell with duplex number; b, synapsis; c, d, e, f, first spermato- cytes with haploid number of chromosomes; g, first spermatocyte division, sex chromosomes (below) in advance of others; h, two polar plates of later stage; 7, first division completed; 7, second spermatocyte with 23 chromo- somes; k, second spermatocyte with 24 chromosomes; /, second spermato- cyte division; m, two polar plates of later stage. 248 HEREDITY AND SEX chromosomes, and two containing 23 chromosomes ; all four sperms having come from the same spermato- gonial cell (Fig. 121). In the female von Winiwarter had difficulty in deter- mining the number of chromosomes present. His - Sex delermtination tn Man (, Mintwarcler J je Se ‘ : A® . iat Ge | o~ . ~ } = / a4 es ie A \ NAGE Ne . 24 / pe r , 2+ , > ee, Z8o | ps 200 tHe AT : nye? UAE ERO es A A Nat I oe es | 234K if Sieh a eae g ase | micnurennees er jis, \ wy et Cr wy ) { =e ay ey far 3 NZ, a . z / : hem. li Pinay, 23 23 “RBtX- ‘ Woh : ‘onvmeenttoP™ 23 ’ ~\ . 5 ‘ j r/ pes ‘ B . \| / Af / yee f Y ie \ yee Foe) SIO o . el E 23 F Fie. 121. — Diagram of human spermatogenesis. A, spermatogonial cell with 47 chromosomes; B, first spermatocyte with reduced haploid number and sex chromosome (open circle) ; C, first division; D, two resulting cells = second spermatocytes; FE, division of second spermatocytes; F, four resulting spermatozoa, two female-producing (above), two male-produc- ing (below). best counts gave 48 chromosomes for the full or duplex number. These observations fit in with the results from the male. 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