Ipli'i- Hi i'UU iJIlMf- ii!iij^«''i'':^" !!'! t ;; tt I if u ' 1 ■ f 'i 1 1 ( t* «i;i! -li'< •"■ :!!•''(; t.'O iMhr liiiliiliiii i! = ,t •li!;t.!s«i;!= I'.t. C-J Columbia Sanifaersitu Hecturcs HEREDITY AND SEX THE JESUP LECTURES 1913 COLUMBIA UNIVERSITY PRESS SALES AGENTS NEW YORK: LEMCKE & BUECHNER 30-32 West 2Tth Street LONDON : HUMPHREY MILFORD Amen Coenek, E.G. TORONTO : HUMPHREY MILFORD 25 Richmond St., W. COLUMBIA UNIVERSITY LECTURES HEREDITY AND SEX 4s- 1 BY THOMAS HUNT MORGAN, Ph.D. PROFESSOR OF EXPERIMENTAL ZOOLOGY IN COLUMBIA UNIVERSITY SECOND {REVISED) EDITION m[t?A COLUMBIA UNIVERSITY PRESS 1914 All rights reserved COPYKIGHT, 1913, By COLUMBIA UNIVERSITY PRESS. Set up and electrotyped. Published November, 1913. Narfajooli i^rcgg J. S. Gushing Co. — Berwick & Smith Co. Norwood, Mass., U.S.A. 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. A hypercritical spirit in science does not always mean greater profundity, nor is our attitude toward science more correct because we are unduly vi 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 zoologists 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 PAGES v-vi CHAPTER I THE EVOLUTION OF SEX 1. Reproduction, a Distinctive Feature of Living Things 2. The "Meaning" of Sexual Reproduction 3. The Body and the Germ-plasm 4. The Early Isolation of the Germ-cells 5. The Appearance of the Accessory Organ Reproduction 6. The Secondary Sexual Characters 7. The Sexual Instincts .... s of . 1-4 . 4-1.5 15-19 20-23 23-26 26-31 31-34 CHAPTER II THE MECHANISM OF SEX-DETERMINATION 1. The Maturation of the Egg and the Sperm . 35-40 2. The Cytological Evidence 40-54 a. Protenor ........ 41-44 b. Lygseus 44-46 c. Oncopeltus ........ 46-48 d. Ascaris . . - 49-52 e. Aphids and Phylloxerans 52-54 3. The Experimental Evidence 55-72 a. The Experiments on Sea-urchins' Eggs . . 55-63 b. The Evidence from Sex-linked Inheritance . 63-72 vii viii TABLE OF CONTENTS CHAPTER III THE MENDELIAN PRINCIPLES OF HEREDITY AND THEIR BEARING ON SEX 1. Mendel's Discoveries 2. The Heredity op^ One Pair of Characters . 3. The Heredity of a Sex-linked Character . 4. The Heredity of Two Pairs of Characters 5. The Heredity of Two Sex-linked Characters 6. A Theory of Linkage 7. Three Sex-linked Factors .... PAGES 73-7.5 75-80 80-84 84-88 88-93 93-97 98-100 CHAPTER IV I 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 Birds . 3. Operations on Amphibia 4. Internal Secretions . 5. Operations on Insects 6. Parasitic Castration of Crustacea 142-144 145-146 146-147 148-1.55 155-1.58 TABLE OF CONTENTS ix CHAPTER VI GYNANDROMORPHISM, HERMAPHRODITISM, PARTHENOGENESIS, AND SEX PAGE 1. gynandromorphism 161-167 2. Hermaphroditism 167-173 3. Parthenogenesis 173-188 4. Artificial Parthenogenesis 188-193 CHAPTER VII FERTILITY 1. Inbreeding 194-199 2. Cross-breeding 200-207 3. Sexual Reproduction in Paramcecium . . . 207-211 4. Theories of Fertility 211-219 CHAPTER VIII SPECIAL CASES OF SEX-INHERITANCE 1. Sex in Bees 220-221 2. A Sex-linked Lethal Factor 221-223 3. Non-disjunction of the Sex-chromosomes . . 223-224 4. The Vanishing Males of the Nematodes . . 224-225 5. Sex-ratios in Hybrid Birds and in Crossed Races in Man 225-227 6. Sex-ratios in Frogs 228-229 7. Sex-ratios in Man 229-232 8. The Abandoned View that External Conditions Determine Sex 232-236 9. Sex-determination in Man . . . - . 236-249 BIBLIOGRHAPHY 251-278 INDEX . . 279-282 HEREDITY AND SEX CHAPTER I 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 compHcated 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 2 HEREDITY AND SEX females as the actors 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 ten 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 Q}/2 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- mcecium 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 utihzed 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 egg begins its development it must be fertiUzed. 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 hterature deahng 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 5 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 Fig. 1. — Union of two individuals (Stephanosphcera pluvialis) to form a single individual. (After Doflein.) 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 paramcecium. 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 paramcecia 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 paramoecium. 6 HEREDITY AND SEX Fig. 2. — Conjugation in Paramoecium. 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 E; 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 (/) with eight micronuclei are shown, by means of which a redistribution of the eight micronuclei takes place. See also Fig. 100. THE EVOLUTION OF SEX I 1 E YiG 3 — The nuclei of two individuals of paramoecium 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, i.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. Biitschli 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 insuflfi- 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. Biitschli'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 paramoecia 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 is that a recombination of the hereditary traits occurs as shown in the diagram, Fig. 3 and 4. Some Fig. 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 paramcecium 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- cal equilibrium of paramoecium 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. A heterozygous or hybrid con- stitution makes this possible, and is more likely to perpetuate itself in the long run than a homozygous race that is from the nature of the case suited to a more limited range of external conditions. What bearing has this conclusion on the problem of the evolution of sex and of sexual reproduction ? This is a question that is certain to be asked. I am not sure that it is wise to try to answer it at present, in the first place because of the uncertainty about the conclusions themselves, and in the next place, because, personally, I think it very unfair and often very unfor- THE EVOLUTION OF SEX 13 tunate to measure the importance of every result by its relation to the theory of evolution. But with this understanding I may venture upon a few suggestions. If a variation should arise in a hermaphroditic species (already reproducing sexually) that made cross- fertilization more likely than self-fertilization, and if, as a rule, the hybrid condition (however this may be explained) is more vigorous in the sense that it leaves more offspring, such a variation would survive, other things being equal. But the estabhshment of the contrivance in the species by means of which it is more likely to cross- fertihze, might in another sense act as a drawback. Should weak individuals appear, they, too, may be perpetuated, for on crossing, their weakness is concealed and their offspring are vigorous owing to their hybrid condition. The race will be the loser in so far as re- cessive or weak combinations will continue to appear, as they do in many small communities that have some deficiency in their race ; but it is a question whether the vigor that comes from mixing may not more than com- pensate for the loss due to the continual appearance of weakened individuals. This argument appUes to a supposed advantage within the species. But recombination of what already exists will not lead to the development of anything that is essentially new. Evolution, however, is con- cerned with the appearance and maintenance of new characters. Admitting that sexual reproduction proved an advantage to species, and especially so when com- bined with a better chance of cross-fertilization, the machinery would be at hand by means of which any 14 HEREDITY AND SEX new character that appeared would be grafted, so to speak, on to the body of the species in which it appeared. Once introduced it would be brought into combination with all the possible combinations, or races, already existing within the species. Some of the hybrid com- binations thus formed might be very vigorous and would survive. This reasoning, while hypothetical, and, per- haps not convincing, points at least to a way in which new varieties may become incorporated into the body of a species and assist in the process of evolution. It might be argued against this view that the same end would be gained, if a new advantageous variation arose in a species that propagated by non-sexual methods or in a species that propagated by self-fertili- zation. The offspring of such individuals would trans- mit their new character more directly to the offspring. Evolution may, of course, at times have come about in this way, and it is known that in many plants self- fertilization is largely or exclusively followed. But in a species in which cross-fertilization was the estab- lished means of propagation, the new character would be brought into relation with all the other variations that are found in the component races and increase thereby its chances of favorable combinations. We have in recent years come to see that a new heritable character is not lost by crossing, or even weakened by ^'blending," as was formerly supposed to be the case; hence no loss to the character itself will result in the union with other strains, or races, within the species. If then we cannot explain the origin of sexual re- production by means of the theory of evolution, we THE EVOLUTION OF SEX 15 can at least see how the process once begun might be utihzed in the building up of new combinations ; and to-day evolution has come to mean not so much a study of the origination of new characters as the method by which new characters become estabhshed after they have appeared. THE BODY AND THE GERM-PLASM As I have said, it is not unusual to speak of the uni- cellular animals and plants as sexual individuals, and where one of them is larger than the other it is some- times called the female and the smaller the male. But in many-celled animals we mean by sex something different, for the term appHes to the body or soma, and not to the reproductive cells at all. The reproductive cells are eggs and sperm. " It leads to a good deal of confusion to speak of the reproductive cells as male and female. In the next chapter it will be pointed out that the eggs and sperm carry certain materials ; and that certain combinations of these materials, after fer- tilization has occurred, produce females ; other combi- nations produce males ; but males and females, as such, do not exist until after fertilization has taken place. The first step, then, in the evolution of sex was taken when colonies of many cells appeared. We find a division of labor in these many-celled organisms ; the germ-cells are hidden away inside and are kept apart from the wear and tear of life. Their maintenance and protection are taken over by the other cells of the colony. Even among the simplest colonial forms we find that some colonies become specialized for the pro- duction of small, active germ-cells. These colonies 16 HEREDITY AND SEX are called the males, or sperm-producing colonies. The other colonies specialize to produce larger germ-cells — the eggs. These colonies are called females or egg-pro- ducing colonies. Sex has appeared in the living world. To-day we are only beginning to appreciate the far- reaching significance of this separation into the immor- tal germ-cells and the mortal body, for there emerges the possibility of endless relations between the body on the one hand and the germ-cells on the other. What- ever the body shows in the way of new characters or new ways of reacting must somehow be represented in the germ-cells if such characters are to be perpetu- ated. The germ-cells show no visible modification to represent their potential characters. Hence the classi- cal conundrum — whether the hen appeared before the egg, or the egg before the hen ? Modern biology has answered the question with some assurance. The egg came first, the hen afterwards, we answer dogmati- cally, because we can understand how any change in the egg will show itself in the next generation — in the new hen, for instance ; but despite a vast amount of arguing no one has shown how a new hen could get her newness into the old-fashioned eggs. Few biological questions have been more combated than this attempt to isolate the germ-tract from the influence of the body. Nussbaum was amongst the first, if not the first, to draw attention to this distinc- tion, but the credit of pointing out its importance is generally given to Weismann, whose fascinating specu- lations start from this idea. For Weismann, the germ- cells are immortal — the soma alone has the stigma of death upon it. Each generation hands to the next THE EVOLUTION OF SEX 17 one the immortal stream unmodified by the experience of the body. What we call the individual, male or female, is the protecting husk. In a sense the body is transient — temporary. Its chief ^^ purpose" is not its individual life, so much as its power to support and carry to the next point the all important reproductive material. Modern research has gone far towards establishing Weismann's claims in this regard. It is true that the germ-plasm must sometimes change — otherwise there could be no evolution. But the evidence that the germ- plasm responds directly to the experiences of the body has no substantial evidence in its support. I know, of course, that the whole Lamarckian school rests its argument on the assumption that the germ-plasm re- sponds to all profound changes in the soma ; but despite the very large literature that has grown up dealing with this matter, proof is still lacking. And there is abun- dant evidence to the contrary. On the other hand, there is evidence to show that the germ-plasm does sometimes change or is changed. Weismann's attempt to refer all such changes to recom- binations of internal factors in the germ-plasm it- self has not met with much success. Admitting that new combinations may be brought about in this way, as explained for paramoecium, yet it seems un- likely that the entire process of evolution could have resulted by recombining what already existed ; for it would mean, if taken at its face value, that by re- combination of the differences already present in the first living material, all of the higher animals and plants were foreordained. In some way, therefore, the germ- 18 HEREDITY AND SEX plasm must have changed. We have then the alter- natives. Is there some internal, initial or driving im- pulse that has led to the process of evolution ? Or has the environment brought about changes in the germ- plasm ? We can only reply that the assumption of an ;:^:^ Fig. 5. — Schematic representation of the processes occurring during the fertilization and subsequent segmentation of the ovum. (Boveri, from Howell.) internal force puts the problem beyond the field of scientific explanation. On the other hand, there is a small amount of evidence, very incomplete and in- sufficient at present, to show that changes in the en- vironment reach through the soma and modify the germinal material. THE EVOLUTION OF SEX 19 It would take us too far from our immediate subject to attempt to discuss this matter, but it has been nec- essary to refer to it in passing, for it lies at the founda- tion of all questions of heredity and even involves, as we shall see later, the question of heredity of sex. This brings us back once more to the provisional conclusion we reached in connection with the experi- ments on paramoecium. When the egg is fertilized by the sperm. Fig. 5, the result is essentially the same as that which takes place when two paramcecia fer- tilize each other. The sperm brings into the egg a nucleus that combines with the egg-nucleus. The new individual is formed by recombining the hereditary traits of its two parents. It is evident that fertilization accomplishes the same result as conjugation. If our conclusion for paramoe- cium holds we can understand how animals and plants with eggs and sperm may better readjust themselves now to this, now to that environment, within certain limits. But we cannot conclude, as I have said, that this process can make any permanent contribution to evolu- tion. It is true that Weismann has advanced the hy- pothesis that such recombinations furnish the materials for evolution, but as I have said there is no evidence that supports or even makes plausible his contention. I bring up again this point to emphasize that while the conclusion we arrived at — a provisional conclusion at best — may help us to understand how sexual repro- duction might be beneficial to a species in maintaining itself, it cannot be utilized to explain the progressive advances that we must believe to have taken place during evolution. 20 HEREDITY AND SEX THE EARLY ISOLATION OF THE GERM-CELLS There is much evidence to show that the germ-cells appear very early in the development of the individual when they are set aside from the cells that differentiate into the body cells. This need not mean that the germ- cells have remained unmodified, although this is at s — ^ s Fig. 6. — Chromatin diminution and origin of the germ-cells in Ascaris. (After Boveri.) THE EVOLUTION OF SEX 21 first sight the most natural interpretation. It might be said, indeed, that they are among the first cells to differentiate, but only in the sense that they specialize, as germ-cells. Fig. 7. — Origin of germ-cells in Sagitta. Korschelt and Heider.) (From In a parasitic worm, ascaris, one of the first four cells divides differently from the other three cells. As seen in Fig. 6, this cell retains at its division all of its chromatin material, while in the other three cells some of the chromatin is thrown out into the cell-plasm. The •• ^ ■ * ^z H;^' ^ ^^ Si- o Fig. 8. — Origin of germ-cells in Miastor. Note small black proto- plasmic area at bottom of egg into which one of the migrating segmentation nuclei moves to produce the germ-cells. (After Kahle.) 22 HEREDITY AND SEX single cell that retains all of the chromatin in its nucleus gives rise to the germ-cells. In a marine worm-like form, sagitta, two cells can easily be distinguished from the other cells in the wall of the digestive tract (Fig. 7). They leave their first posi- tion and move into the interior of the body, where they produce the ovary and testes. Lepldosteus Lepldosleus Chrysemys SPeriph. End. XVil.Cntl Fig. 9. — Origin of germ-cells in certain vertebrates, viz. turtle, frog, gar-pike and bow-fin. The germ-cells as darker cells are seen migrating from the digestive tract (endoderm). (After Allen.) In several of the insects it has been shown that at a very early stage in the segmentation, one, or a few cells at most, lying at one end of the egg develop almost in- dependently of the rest of the embryo (Fig. 8). Later they are drawn into the interior, and take up their final location, where they give rise to the germ-cells. Even in the vertebrates, where, according to the THE EVOLUTION OF SEX 23 earlier accounts, the germ-cells were described as appear- ing late in embryonic development, it has been shown that the germ-cells can be detected at a very early stage in the walls of the digestive tract (Fig. 9). Thence they migrate to their definitive position, and give rise to the cells from which the eggs or the sperm arise. The germ-cells are in fact often the earliest cells to specialize in the sense that they are set aside from the other cells that produce the soma or body of the in- dividual. THE APPEARANCE OF THE ACCESSORY ORGANS OF REPRODUCTION As animals became larger the problem of setting free the germ-cells was a matter of great importance. Sys- tems of outlets arose — the organism became piped, as it were. In the lower animals the germ-cells are brought to the surface and set free directly, and fertilization is a question of the chance meeting of sperm and egg ; for there is practically no evidence to show that the sperm is attracted to the egg and much evidence that it is not. Later, the copulatory organs were evolved in all the higher groups of animals by means of which the sperm of the male is transferred directly to the female. This makes more certain the fertilization of the egg. In the moUusks, in the insects and crustaceans, and in the vertebrates the organs of copulation serve to hold the individuals together during the act of mating, and at the same time serve to transfer the semen of the male to the oviduct, or to special receptacles of the female. Highly elaborated systems of organs and special instincts, no less elaborate, serve to make the 24 HEREDITY AND SEX union possible. In some types mating must occur for each output of eggs, but in other cases the sperm is stored up in special receptacles connected with the ducts of the female. From these receptacles a few sperm at a time may be set free to fertilize each egg as it passes the opening of the receptaculum. In the queen bee enough sperm is stored up to last the queen for five or six years and enough to fertilize a million eggs. ^' / !*•■■ ? Fig. 10. — Squid : Two upper right-hand figures illustrate two methods of copulation. Lower right-hand figure dissected to show spermatophore placed in mantle cavity of female. Left-hand figure (below), spermatophore pocket behind mouth of male; upper figure, section of same. (After Drew.) There are a few cases where the transfer from the male to the female is brought about in a different way. The most striking cases are those of the squids and octopi, and of the spiders. In the squid, the male and female interlock arms (Fig. 10). The male takes the packets of sperm (that are emitted at this time from the sperm-duct) by means of a special arm, and transfers the packets either to a THE EVOLUTION OF SEX 25 special receptacle within the circle of arms of the female, or plants them within the mantle chamber itself of the female. Each packet of spermatozoa is contained in a long tube. On coming in contact with sea water the tube everts at one end, and allows the sperm to escape. Fig. 11. — Octopus, male showing hectocotyl arm {ha). Cop- ulation (below), small male, A; large female, B. After separation the female deposits her strings of eggs, which are fertiUzed by the sperm escaping from the spermatophores. In octopus and its allies, one arm, that is used to transfer the spermatophores, is specially modified at the breeding season (Fig. 11). 26 HEREDITY AND SEX This arm is inserted by the male, as shown in the figure, within the mantle chamber of the female. In some species. Argonaut a argo for instance (Fig. 12), the arm Fig. 12. — Argonauta showing developing (A) and developed (B) hectocotyl arm, which, after being charged with spermatophores, is left in mantle of female. is broken off, and remains attached by its suckers in- side the mantle of the female. The eggs are later fer- tilized by sperm set free from this '^hectocotylized " arm. THE SECONDARY SEXUAL CHARACTERS In the most highly evolved stages in the evolution of sex a new kind of character makes its appearance. This is the secondary sexual character. In most cases such characters are more elaborate in the male, but occasionally in the female. They are the most aston- ishing thing that nature has done : brilliant colors, plumes, combs, wattles, and spurs, scent glands (pleas- ant and unpleasant) ; red spots, yellow spots, green spots, topknots and tails, horns, lanterns for the dark, songs, bowlings, dances and tourneys — a medley of odds and ends. The most familiar examples of these characters are found in vertebrates and insects, while in lower forms THE EVOLUTION OF SEX 27 they are rare or absent altogether. In mammals the horns of the male stag are excellent examples of second- ary sexual characters. The male sea cow is much greater in size than the female, and possesses long tusks. The mane of the lion is absent in the lioness. Fig. 13. — Great bird of Paradise, male and female. (After Elliot.) In birds there are many cases in which the sexes differ in color (Figs. 13 and 14). The male is often more brilliantly colored than the female and in other cases the nuptial plumage of the male is quite different from the plumage of the female. For example, the black and yellow colors of the male bobolink are in striking contrast with the brown-streaked female (Fig. 15). The male scarlet tanager has a fiery red plumage with black wings, while the female is olive green. The male 28 HEREDITY AND SEX of the mallard duck has a green head and a reddish breast (Fig. 16), while the female is streaked with brown. In insects the males of some species of beetles have horns on the head that are lacking in the female (Fig. 17). The males of many species of butterflies are col- ored differently from the females. / Fig. 14. — White-booted humming bird, two males and one female. (After Gould.) The phosphorescent organ of our common firefly, Photinus pyralis, is a beautiful illustration of a second- ary sexual character. On the under surface of the male there are two bands and of the female there is a single band that can be illuminated (Fig. 18). At night the males leave their concealment and fly about. A little later the females ascend to the tops of blades of grass THE EVOLUTION OF SEX 29 Fig. 15. — Male and female bobolink. (From " Bird Lore.") Fig. 16. — Male and fcmule mallard duck. (From " Bird Lore.") 30 HEREDITY AND SEX and remain there without glowing. A male passes by and flashes his Hght ; the female flashes back. In- stantly he turns in his course to the spot whence the signal came and alights. He signals again. She re- plies. He ascends the blade, and if he cannot find her, he signals again and she responds. The signals con- FiG. 17. — Male and female Hercules beetle. (After Kingsley.) tinue until the female is found, and the drama of sex is finished. Mast has recently shown that the female firefly does more than simply respond to the signal of the male. If a male flies above and to the right of the female, she bends her abdomen so that its ventral surface is turned upward and to the right. If the male is above and to the left, the light is turned in this direction. If the male THE EVOLUTION OF SEX 31 is directly above, the abdomen of the female is twisted almost upward. But if the male is below her, she emits her light without turning the body. In the firefly the evidence that the phosphorescent organ is of use in bringing the sexes together seems well estabUshed. Fig. 18. — Male and female firefly. Whether all secondary sexual organs are useful in mating is a question that must be referred to a later chapter. THE SEXUAL INSTINCTS Side by side with the evolution of these many kinds of structural difference the sexual instincts have evolved. It is only in the lowest forms that the meeting of the egg and sperm is left to chance. The instincts that bring the males and females together at the mating season, the behavior of the individuals at this time in 32 HEREDITY AND SEX relation to each other, forms one of the most curious chapters in the evolution of sex, for it involves court- ship between the males and females ; the pairing or union of the sexes and subsequently the building of the nest, the care, the protection and feeding of the young, by one or both parents. The origin of these types of behavior is part of the process of evolution of sex ; the manner of their transmission in heredity and their segregation according to sex is one of the most difficult questions in heredity — one about which noth- ing was known until within recent years, when a beginning at least has been made. A few samples taken almost at random will illustrate some of the familiar features in the psychology of sex. Birds have evolved some of the most complicated types of courtship that are known. It is in this group, too, as we have seen, that the development of secondary sexual characters has reached perhaps its highest types. But it is not necessarily in the species that have the most striking differences between the sexes that the courtship is most elaborate. In pigeons and their allies, for example, the courtship is prolonged and elab- orate, yet the males and females are externally al- most indistinguishable ; while in the barnyard fowl and in ducks the process is relatively simple, yet chanti- cleer is notoriously overdressed. Even in forms so simply organized as the fishes it is known that the sexual instincts are well developed. In the common minnow, fundulus, the males develop in the breeding season elaborate systems of tactile organs. The male swims by the side of the female, pressing his body against her side, which causes her to set free THE EVOLUTION OF SEX 33 a few eggs. At the same time the male sets free the sperm, thereby increasing the chance that some of the spermatozoa will reach the egg. In bees, the sexual life of the hive is highly special- ized. Mating never occurs in the hive, but when the young queen takes her nuptial flight she is followed by the drones that up to this have led an indolent and use- less life in the colony. Mating occurs high in the air. The queen goes to the new nest and is followed by a swarm of workers who construct for her a new home. Here she remains for the rest of her life, fed and cared for by the workers, who give her the most assiduous attention — an attention that might be compared to courting were it not that the workers are not males but only immature females. The occurrence of these mstincts in the workers that never leave or rarely at least leave offspring of their own is a special field of heredity about which we can do little more than specu- late. This much, however, may be hazarded. The inheritance of the queen and of the worker is the same. We know from experimental evidence that the amount of food given to the young grub, when it hatches from the egg, is the external agent that makes the grub a queen or a worker. In the worker the sex glands are httle developed. Possibly their failure to develop may in part account for the different behavior of the workers and of the queen. I shall devote a special chapter to this question of the influence of the secretions of the sex glands or reproductive organs on the character of the body. We shall see that in some animals at least an important relation exists between them. In the spiders the mating presents a strange spectacle. 34 HEREDITY AND SEX Let us follow Montgomery's careful observations on Phidippus purpuratus. The male spun a small web of threads from the floor to one side of his cage at an angle of 45°. ''Four minutes later he deposited a minute drop of sperm on it, barely visible to the naked eye ; then extending his body over the web reached his palpi downwards and backwards, applying them al- ternately against the drop ; the palpal organs were pressed, not against the free surface of the drop, but against the other side of the web." Later, a minute drop of sperm is found sticking to the apex of one of the palpi. In 1678 Lister had shown that the male applies his palpi to the genital aperture of the female ; but not until 1843 was it found by Menge that the palpi carry the sperm drop. In man, courtship may be an involved affair. Much of our literature revolves about this period, while paint- ing and sculpture take physical beauty as their theme. Unsatiated with the natural differences that distinguish the sexes, man adds personal adornment which reaches its climax in the period of courtship, and leaves a lasting impression on the costuming of the sexes. Nowhere in the animal kingdom do we find such a mighty display ; and . clothes as ornaments excel the most elaborate developments of secondary sexual char- acters of creatures lower in the scale. I have sketched in briefest outline some of the gen- eral and more familiar aspects of sex and the evolution of the sexes. In the chapters that follow we shall take up in greater detail many of the problems that have been only touched upon here. CHAPTER II The Mechanism of Sex-determination In many species of animals and plants two kinds of individuals are produced in every generation. This process occurs with such regularity and persistence that our minds naturally seek some mechanism, some sort of orderly machinery, by which this condition is brought about. Yet from the time of Aristotle almost to the present day the problem has baffled completely all attempts at its solution. However, the solution is very simple. Now that we hold the situation in our grasp, it seems surprising that no one was keen enough to deduce it by purely theoretical reasoning. At least the general principles involved might have been de- duced, although we can see that without an intimate knowledge of the changes that take place in the germ- cells the actual mechanism could never have been foretold. The bodies of animals and plants are composed of millions of protoplasm-fifled compartments that are called cells. In the middle of each cell there is a sphere, or nucleus, containing filaments called chromosomes (Fig. 5). At each division of a cell the wall of the nucleus is absorbed, and the thread-like chromosomes contract into rod-shaped, or rounded bodies (Fig. 6). Each chromosome sphts lengthwise into halves; the halves 35 36 HEREDITY AND SEX are brought into relation with a spindle-shaped system of lines, and move apart along these lines to opposite sides of the cell. The protoplasm of the cell next con- stricts to produce two daughter cells, each containing a group of daughter chromosomes. Fig. 19. — Fertilization and polar-body formation of Nereis. The four smaller figures show entrance of sperm. The extrusion of the first polar body is shown in lower left-hand figure and of the second polar body in the two large right-hand figures. The last three also show the formation of the sperm asters, which is the beginning of the first cleavage spindle in the egg. (After F. R. Lillie.) The egg is also a cell, and in its earlier stages contains the same number of chromosomes as do the other cells of the body; but after two pecuUar divisions that take place at maturation the number of the chromosomes is reduced to half. THE MECHANISM OF SEX-DETERMINATION 37 But before this time the egg-cells divide, like all the other cells of the body. In this way a large number of eggs is produced. After a time they cease to divide and begin to grow larger, laying up yolk and other materials. At this time, the chromosomes unite in pairs, so that their number seems to be reduced to half the original number. At the final stage in the matura- tion of the egg, two peculiar divisions take place that involve the formation of two minute cells given off at one pole — the polar bodies. In some eggs, as in the sea urchin, the polar bodies are given off while the egg is still in the ovary and before fertilization ; in other eggs, as in the frog, one polar body is given off before fertilization, the other after the sperm has entered ; and in other eggs, as in nereis (Fig. 19), both polar bodies are given off after fertilization. The formation of the polar bodies is a true cell- division, but one that is unique in two respects. First, one of the cells is extremely small, as seen in Fig. 19. The smallness is due to the minute amount of protoplasm that it contains. Second, the number of chromosomes at each division is the half or ^'haploid " number. There is much evidence to show that at one or at the other of these two divisions the two chromo- somes that had earlier united are separated, and in this respect this division differs from all other cell-divisions. In consequence, the egg nucleus, that re-forms after the second polar body has be^n produced, contains only half the actual number of chromosomes characteristic of all the other cells of the female. In the formation of the spermatozoa a process takes place almost identical with the process just described 38 HEREDITY AND SEX for the female (Fig. 20). In their earUer history the germ-cells of the male divide with the full number of chromosomes characteristic of the male, which may be one less chromosome than in the female. The early Fig. 20. — A-B, somatic cell division with four chromosomes. C-H, the two matviration divisions to produce the four cells {H) that become spermatozoa. (After Wilson.) germ-cells then cease to divide for a time, and begin to grow, laying up yolk and other materials. At this time the chromosomes unite in pairs, so that the num- ber appears to be reduced to half. Later two divisions occur (Fig. 20, D-H), in one of which the united chro- mosomes separate. The male germ-cells differ, how- THE MECHANISM OF SEX-DETERMINATION 39 ever, from the female, in that at each of these two di- visions the cells are equal in size. Thus four sperm- cells are produced from each original cell, all four pro- duce tails, and become spermatozoa. At the time of fertilization, when the spermatozoon touches the surface of the egg, the egg pushes out a cone of protoplasm at the point of contact (Fig. 19), and, lending a helping hand, as it were, to the sperm, draws it into the egg. The projecting cone of protoplasm is called the fertilization cone. In a few minutes the head of the sperm has entered. Its tail is often left outside. The head absorbs fluid from the egg and becomes the sperm nucleus, which passes towards the center of the egg. Here it comes to lie by the side of the egg nucleus, and the two fuse. The walls of the com- bined nuclei dissolve away and the chromosomes appear. Half of these are derived from the father through the nucleus of the sperm, and half from the mother through the egg nucleus. If we count the paternal chromosomes, there are half as many of them as there are chromosomes in each cell of the body of the father. Presently I shall point out that this statement is not always true, and on this little fact, that it is not quite true, hangs the whole story of sex-determination. What is the meaning of these curious changes that have taken place in the egg and sperm ? Why has the egg, before developing, twice thrown away its most valuable heritage — its chromatin material? We do not know with certainty, but one consequence at least stands out clearly ! Before the egg gave off its polar bodies it had the full, or diploid, number of chromo- somes. After this event it has only half as many. A 40 HEREDITY AND SEX similar reduction occurs in the sperm, excepting that no chromatin is lost, but is redistributed amongst four spermatozoa. Egg and sperm-nucleus each have in consequence the haploid or half number. By combin- ing they bring up the number to that characteristic of the species. The history of the germ-cells, that we have just traced, is the background of our knowledge of the pro- cess of heredity in so far as observable changes in the germ-cells have been made out. We owe to Weismann more than to any other biologist the realization of the importance of these changes. It is true that Weismann contributed only a part of the actual facts on which the interpretation rests. Many workers, and a few leaders, have laboriously made out the com- plete account. But Weismann, by pointing out the supreme importance of the changes that take place at this time, has furnished a stimulus that has acted like yeast in the minds of less imaginative workers. We are now in a position to apply this knowledge to the interpretation of the mechanism by means of which sex is determined. THE CYTOLOGICAL EVIDENCE If we study by means of modern histological methods the body cells of the male of the insect, Protenor belfragei, we find, when each cell is about to divide, that a group of chromosomes appears like that shown in Fig. 21, A. There are twelve ordinary oval chromo- somes, and one much larger than the rest. This group of chromosomes is characteristic of all divisions of the cells of the body, regardless of whether the cells belong THE MECHANISM OF SEX-DETERMINATION 41 to muscle, skin, gland, ganglion, or connective tissue. The early germ-cells of the male, the so-called ''sper- matogonia," also have this same number. It is not until a later stage in their development that a remarkable change takes place in them. When this change occurs the thread-like chromosomes unite in pairs. This is the synapsis stage — the word means to fuse together. It is the most difficult stage to interpret in the whole history of the germ-cells. In a few forms where the changes that take place have been seen to best advan- tage it is found that chromosomes are in the form of long threads and that these threads unite in pairs to make thicker threads. When the process is completed, we find half as many threads as there were before. This statement is not quite true. In the case of the male protenor, for instance, there are twelve ordinary chromosomes and one large one. The twelve unite in pairs at synapsis, so that there are six double chromo- somes, but the large one has no mate (Fig. 21, B). When the others have united in synapsis, it has taken no part in the process, hence the reduced number of chromosomes in the male is seven — the seventh is the sex chromosome. Two divisions now follow each other in rapid succes- sion (Fig. 21, C, D). In the first division (C) each chromosome divides — seven go to one pole and seven to the other pole. Two cells, the primary spermato- cytes, are produced. Without resting, another divi- sion takes place (D) in each of these two cells. It is the second spermatocyte division. Each of the six ordinary chromosomes divides, but the large sex chro- mosome does not divide, and, lagging behind the others, 42 HEREDITY AND SEX as shown in the figure (D), it passes to one pole. Each secondary spermatocyte produces, therefore, two cells — one with six, the other with seven chromosomes. These cells become spermatozoa (EE'), the ones with seven chromosomes are the female-producing spermatozoa, the ones with six chromosomes are the male-producing Prot&nor -•*. ^^z- A ^ ■.:-:^^!^^4-fti : *^ ^V *^;'> f — FiG. 29a. — 20 and 21, oogonia (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 egg are formed, all the chromosomes divide twice, sp 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 t^cn 3 cTo FC'isf Q^jbcundtbet^fe'. Second Sfiet^naZoti^Xi^. Fig. 30. — Diagram of chromosomes in Phylloxera carycecaulis. 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. THE MECHANISM OF SEX-DETERMINATION 55 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 spermatozoon 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. Fig. 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 p^s^ / @© ' ©© ;©© I @ ■ . ■ ...... / @@ © © © V © © t \^®@ @@/ /®®W:'- ••■/©,©.(£)■.■ .::•■;.■ ®r^\ f @® © \©)@@ ■■■©©>#>:• '■'yW^y .' •■'©©©!■ >"g).®@.^\ .■:v;^j@:;V. V©)©©-' ;•:■:%: .;:v;\ :;■:;•.:©.•-•• ■:@®^ 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. Any cell that does not contain at least these four chromosomes is shaded. One case is present in which all the four 58 HEREDITY AND SEX cells contain a complete assortment. If normal devel- opment depends on an embryo containing in every cell at least one of each kind of chromosome, then in our simple case only one group of four cells has this possibility. Boveri found that such dispermic eggs produce normal embryos very rarely. He calculated what the chance would be when three times 18 chromosomes are involved. The chance for normal development is probably not once in 10,000 times. He isolated many dispermic eggs and found that only one in 1,500 of the tetrad type developed normally. Boveri went still further in his analysis of the prob- lem. It had been shown for normal eggs that if at the two-celled stage the cells are separated, each forms a perfect embryo. This is also true for each of the first four cells of the normal egg. Boveri separated the four cells of dispermic eggs and found that the quadrants not infrequently developed normally. This is what we should anticipate if those cells can develop that contain one of each kind of chromosome. The evidence furnishes strong support of the view that the chromosomes are different from each other, and that one of each kind is necessary if development is to take place normally. The evidence that Baltzer has brought forward is also derived from a study of sea-urchin eggs. It is possible to fertilize the eggs of one species with sperm of another species. The hybridizing is greatly helped by the addition of a little alkali to the sea water. Baltzer made combinations between four species of THE MECHANISM OF SEX-DETERMINATION 59 sea-urchins. We may take one cross as typical. When eggs of strongylocentrotus are fertiUzed with sperm of sphserechinus, it is found at the first division of the egg that, while some of the chromosomes divide and pass normally to the two poles, other chromosomes remain in place, or become scattered irregularly between the two poles, as shown in Fig. 33. When the division \Mn j//'iV^ ^,^, ± x» I /rij'')»» ,/>;<.;'!'j;f\ '^W'^ h^,h " ) fin 54 . ''//; iiHW Fig. 33. — 1 and la, chromosomes in the normal first cleavage spindle of Sphaerechinus ; 2, equatorial plates of two-cell stage of same ; 3-3a, hybrid, Sphserechinus by Strongylocentrotus, spindle at two-cell stage ; 4-4a, same equatorial plates; 5-5a, hybrid, Strong, by Sphaer., cleavage spindle in telo- phase ; 6, next stage of last ; 7, same, two-cell stage ; 8, same, later ; 9, same, four-cell stage ; 10, same, equatorial plate in two-cell stage (12 chromosomes) ; 11, same, from later stage, 24 chromosomes. (After Baltzer.) is completed, some of these chromosomes are found outside of the two main nuclei. They often appear as irregular granules, and show signs of degeneration. They are still present as defiilite masses after the next division, but seem to take no further part in the de- velopment. Baltzer has attempted to count the number of chro- mosomes in the nuclei of these hybrid embryos. The 60 HEREDITY AND SEX number is found to be about twenty-one. The maternal egg nucleus contains eighteen chromosomes. It appears that only three of the paternal chromosomes have succeeded in getting into the regular cycle — fifteen of them have degenerated. Baltzer thinks that the egg acts injuriously in this case on the chromosomes of foreign origin, especially on the fifteen that degenerate, so that they are elim- inated from the normal process. The embryos that develop from these eggs are often abnormal. A few develop as far as the pluteus stage, when a skeleton appears that is very characteristic for each species of sea-urchin. The plutei of these hybrids are entirely maternal. This means that they are exactly like the plutei of the species to which the mother belongs. The conclusion is obvious. The sperm of sphserechi- nus has started the process of development, but has produced no other effect, or has at most only slightly affected the character of the offspring. It is reason- able to suppose that this is because of the elimination of the paternal chromosomes, although the evidence is not absolutely convincing. Let us now examine the reciprocal cross. When the eggs of sphserechinus are fertilized by the sperm of strongylocentrotus, the division of the egg and of the chromosomes is entirely normal. All the chromosomes divide and pass to the poles of the spindle. The total number (36) must, therefore, exist in each cell, although in this case they were not actually counted. The pluteus that develops has peculiarities of both maternal and paternal types. It is hybrid in structure. THE MECHANISM OF SEX-DETERMINATION 61 Both parents have contributed to its formation. It is not going far from the evidence to infer that the hybrid character is due to both sets of chromosomes being present in all of the cells. ■U rP A\o o/ Fig. 34. ^ 1. The chromosomes of the egg lie in the equator of the spindle, the chromosomes of the sperm lie at one side. 2. A later stage, showing all the paternal chromosomes passing to one pole. 3 (to the right). A later stage, a condition like the last. There is also a supernumerary sperm in the egg (to left, in another section.) 4. Same condition as last. 5. Plu- teus larva that is purely maternal on one side and hybrid on the other. (After Herbst.) The evidence that Herbst.has brought forward is of a somewhat different kind. It supplements Baltzer's evidence and makes more probable the view that the chromosomes are essential for the development of the characters of the individual. 62 HEREDITY AND SEX Herbst put the eggs of sphserechinus into sea water to which a httle valerianic acid had been added. This is one of the many methods that Loeb has discovered by which the egg may be induced to develop parthe- nogenetically, i.e. without the intervention of the sper- matozoon. After five minutes the eggs were removed to pure sea water and the sperm of another species, stron- gylocentrotus, was added. The sperm penetrated some of the eggs. The eggs had already begun to undergo the changes that lead to division of the cell — the sperm entered ten minutes late. The egg proceeded to divide, the sperm failed to keep pace, and fell behind. Consequently, as shown in Fig. 34, the paternal chromosomes fail to reach the poles when the nuclei are re-formed there. The paternal chromosomes form a nucleus of their own which comes to lie in one of the two cells. In consequence one cell has a nucleus that contains only the maternal chromosomes ; the other cell contains two nuclei, one maternal and the other paternal. In later development the paternal nucleus becomes incorporated with the maternal nucleus of its cell. There is produced an embryo which is maternal on one side and hybrid on the other. Herbst found in fact that half-and-half plutei were not rare under the conditions of his experiment. This evidence is almost convincing, I think, in favor of the view that the chromosomes are the es- sential bearers of the hereditary qualities. For in this case, whether the protoplasm of the embryo comes from the egg or the sperm also, the fact re- mains that the half with double nuclei is hybrid. Even if the spermatozoon brought in some proto- THE MECHANISM OF SEX-DETERMINATION 63 plasm, there is no reason to suppose that it would be distributed in the same way as are the paternal chromosomes. EVIDENCE FROM SEX-LINKED INHERITANCE The experimental evidence based on sex-linked in- heritance may be illustrated by the following examples. The eyes of the wild fruit-fly, Drosophila ampe- lophila, are red. In my cultures a male appeared chat had white eyes. He was mated to a red-eyed female. The offspring were all red-eyed — both males and females (Fig. 35). These were inbred and produced in the next generation red-eyed females, red-eyed males, and white-eyed males (Fig. 35). There were no white- eyed females. The white-eyed grandfather had trans- mitted white eyes to half of his grandsons but to none of his granddaughters. Equally important are the numerical proportions in which the colors appear in the grandchildren. There are as many females as the two classes of males taken together; half of the males have red eyes and half have white eyes. The proportions are therefore 50 % red females, 25 % red males, 25 % white males. Only white-eyed males had appeared at this time. It may seem that the eye color is confined to the male sex. Hence the origin of the term sex-limited inheri- tance for cases like this. But white-eyed females may be produced easily. If certain of the red-eyed grand- daughters are bred to these white-eyed males, both white-eyed females and males, and red-eyed females and males, appear (Fig. 37). The white eye is there- fore not sex-limited but sex-linked. 64 HEREDITY AND SEX With these white-eyed females it is possible to make the reciprocal cross (Fig. 36). A white-eyed female was mated to a red-eyed male. All of the daughters had red eyes and all the sons had white eyes. These were then inbred and gave red-eyed males and females, 'fi XX XX ~cr XXXIX Fig. 35. — Sex-linked inheritance of white and red eyes in Drosophila. Parents, white-eyed Z and red-eyed 9 ; F\, red-eyed Z and $ ; F2 red- eyed 9 . red-eyed Z and white-eyed Z • To right of flies the history of the sex chromosomes XX. is shown. The black X carries the factor for red eyes, the open X the factor for white eyes ; the circle stands for no X. and white-eyed males and females in equal numbers (Fig. 36). The heredity of this eye color takes place with the utmost regularity, and the results show that in some way the mechanism that is involved is closely bound up with the mechanism that produces sex. THE MECHANISM OF SEX-DETERMINATION 65 Other combinations give results that are predictable from those just cited. For instance, if the Fi red-eyed female from either of the preceding crosses is mated to a white-eyed male, she produces red-eyed males and females, and white-eyed males and females, as shown in X® r W/f^ ^)XIX( 1 Fig. 36. — Reciprocal cross of Fig. 35. Parents, white-eyed 9 and red-eyed $, (criss-cross inheritance). Fi, red-eyed 9. white-eyed $. F2, red-eyed 9 and $ ; white-eyed 9 and $ . To right, sex chromo- somes (as in Fig. 35). Fig. 37 (upper two lines). If the Fi red-eyed male from the first cross (Fig. 35) is bred to a white-eyed female, he will produce red-eyed daughters and white- eyed sons. Fig. 37 (lower two lines). The same relations may next be illustrated by an- other sex-linked character. 66 HEREDITY AND SEX A male with short or miniature wings appeared in my cultures (Fig. 38). Mated to long-winged females only long-winged offspring were produced. When these were mated to each other, there were produced X X X X Fig. 37. — Upper series, back cross of Fi 9 to white J back cross of Fi red-eyed $ to white 9 • Lower series long-winged females (50%), long-winged males (25%) and miniature-winged males (25%). It is possible to produce, in the way described for eye color, miniature-winged females. When such miniature-winged females are mated to long-winged males, all the daughters have long wings, and all the sons have miniature wings (Fig. 39). If THE MECHANISM OF SEX-DETERMINATION 67 these are now mated, they produce, in equal numbers, long-winged males and females and miniature-winged males and females. The same relations may again be illustrated by body color. XX rr X@ XI XXXIX Fig. 38. — Sex-linked inheritance of short (" miniature") and long wings in Drosophila. Parents, short-winged $ , long-winged 9 . Fi long-winged $ and 9 • Fi long-winged 9 and $ and short-winged $ . Sex chromo- somes to right. Open X carries short wings. A male appeared with yellow wings and body. Mated to wild gray females he produced gray males and females. These mated to each other gave gray females (50%), gray males (25%), and yellow males (25%). As before, yellow females were made up. Mated to gray males they gave gray females and yellow males. 68 HEREDITY AND SEX These inbred gave gray males and females and yellow males and females, in equal numbers. These cases serve to illustrate the regularity of this type of inheritance and its relation to sex. In the fruit fly we have found as many as twenty-five sex-linked X XM ;C^ Ct <£:5< •^ /» u ) t ^ ^/ C ^ 30 ^ ' fA ,Ai.n«. ^Amt M In^ L MatMMI. 1 .. ■ ^t Fig. 69. — Fat-tailed hornless sheep (Ovis aries steatopyga persicci). have well-developed horns. In this case the hereditary factors suffice in themselves for the complete develop- ment of horns, for even after castration the horns de- velop. We have anticipated to some extent the conclusions arrived at by breeding experiments in these races of sheep. The best-known case is that of Wood, who crossed horned Dorsets and hornless Suffolks. As THE EFFECTS OF CASTRATION 137 shown in the picture (Fig. 70) the sons had horns — the daughters lacked them. When these are inbred, their offspring are of four kinds, horned males, hornless males, horned females, hornless females. It seems probable that these four classes appear in the following proportions : Horned c? Hornless 6 Horned ? Hornless ? 3 11 3 The explanation that Bateson and Punnett offer for this case is as follows : The germ-cells of the horned race Fig. 70. — 1, Suffolk (ram), hornless in both sexes; 2, Dorset (ewe), horned in both sexes ; 3, iP'i ram, horned ; 4, Fi ewe, hornless ; 5-8, the four types of F2 ; 5 and 6 are rams, 7 and 8 are ewes. The hornless rams are pure for absence of horns, and the horned ewes are pure for the presence of horns. Figs. 5 and 6 represent lambs. (Bateson, after Wood.) (both male and female) carry the factor for horns (H) ; the germ-cells of the hornless race lack the factor for horns (/i). The female is assumed to be homozygous for the sex factor, i.e. two sex chromosomes (X) are present ; while the male has only one sex chromosome 138 HEREDITY AND SEX carried by the female-producing sperm. The analysis is then as follows: One ''dose" of horns (H) in the male produces horns, but two doses are necessary for the female. Hornless ? hX — hX Horned S H X — H Fi H X h X hornless ? H h X horned 6 Gametes of Fi Eggs HX — hX Sperm H X — hX — H h F2 Females H X H X horned H X h X hornless h X H X hornless h X h X hornless F2 Males H H X horned H h X horned h H X horned h h X hornless As pointed out by Punnett a test of the correctness of this interpretation is found by breeding the Fi hornless female to a hornless male (of a hornless breed) . It is assumed that such a female carries the factors for horns in a heterozygous condition ; if so, then half of her sons should have horns, as the following analysis shows : H X — hX hX — h Fi Hornless ? Hornless 3 h X H X hornless 9 h X h X hornless ? h H X horned c? h h X hornless 3 THE EFFECTS OF CASTRATION 139 '''W -Bw ..GP Fi- Fig. 71. — Upper figure normal male guinea pig (from below), to show mammary glands. Lower figure, a feminized male ; i.e. castrated when three weeks old and pieces of ovaries transplanted beneath the skin, at Ov. 140 HEREDITY AND SEX The actual result conforms to the expectation. The results of both of the experiments are consistent with the view that one factor for horns in the male produces horns, which we may attribute to the combined action of the inherited factor and a secretion from the testes which reenforces the action of the latter. This, how- ever, should be tested by castrating the Fi males. In the females, one factor for horns fails to produce horns, while two factors for horns cause their development. Aside from some of the domesticated animals (horses, cattle, dogs, cats, pigs), the only other mammals on which critical experiments have been made — if we exclude man — are the rat and the guinea pig. The next case is unique in that the ovary was transplanted to a male. Steinach removed the sex glands from the male guinea pig and rat and transplanted into the same animals the ovaries of the female, which established themselves. Their presence brought about remarkable effects on the castrated male. The mammary glands, that are in a rudimentary condition in the male, be- come greatly enlarged (Fig. 71). In the rat the hair assumes the texture of that of the female. The skele- ton is also more like that of the female than the male. The size of the feminized rats and guinea pigs is less than that of normal (or of castrated) males and Hke that of the female (Fig. 72). Finally, in their sexual behavior, the feminized rats were more like females than like males. These cases are important because they are the only ones in which success- ful transplanting of the ovary into a male has been accomplished in vertebrates. THE EFFECTS OF CASTRATION 141 Fig. 72. — Two upper figures, normal male guinea pig to left, M, and his brother, F, to right — a feminized male. Two middle and two lower figures, a normal male at M, and his feminized brother, F. (After Steinach ) 142 HEREDITY AND SEX OPERATIONS ON BIRDS In striking contrast to these results with mammals are those with birds, where in recent years we have gained some definite information concerning the devel- opment of secondary sexual characters. I am fortunate in being able to refer to several cases — the most successful on record — carried out by my friend, H. D. Goodale, at the Carnegie Lab- oratory at Cold Spring Harbor. One ^'case" is that of a female Mallard duck from which the ovary was completely removed when she was a very young bird. Figure 16 illustrates the striking difference between the normal male and the female Mallard. In the spayed female the plumage is like that of the male. Darwin records a case in which a female duck in her old age assumed the characteristics of a male, and similar cases are recorded for pheasants and fowls. Goodale also removed the ovary from very young chicks. He found that the female developed the secondary sexual plumage of the cock. How shall we interpret these cases ? It is clear that the female has the potentiality of producing the full plumage of the male, but she does not do so as long as the ovary is present. The ovary must therefore be supposed to prevent, or inhibit, the development of secondary sexual characters that appear therefore only in the male. The converse operation — the removal of the male glands from the male — is an operation that is very common among poultry men. The birds grow larger and fatter. They are known as capons. In this case THE EFFECTS OF CASTRATION 143 the male assumes his full normal plumage with all of his secondary male sexual characters. It is said that the comb and wattles and to some extent the spurs are less developed in the capon than in the normal male. But aside from this it is quite certain that the de- velopment of the secondary sexual plumage in the (o py Ri e^rrr-tgij'^^- ■ ^^^-/f- — Male and female Seabright. Note short neck feathers and rrr b' *"' T/T '" TI" J^ ^^^ ^^^^^^^^ -^^ the slckle feathers on Journal '0 ^'' ^^""'^ °^ *^^ ^"'^' ^^^^^' "Reliable Poultry male is largely independent of the presence of the sex glands. The method of inheritance of the secondary sexual characters in birds has been httle studied. Daven- port has reported one case, but I am not sure of his in- terpretation. ^ I have begun to study the question by usmg Seabright bantams, in which the male lacks some ' Because it is not evident whether the secondary sexual char- acters as such are involved or only certain general features of coloration. 144 HEREDITY AND SEX of the secondary sexual characters of the domestic races, notably the saddle feathers, as shown in Fig. 73. A male Seabright was mated to a black-breasted game female. The son was hen-feathered and like the Sea- bright father in this respect. Evidently in this case the secondary sexual character in question is dominant and is transmitted from father to son. In the reciprocal cross one hen was obtained which was back-crossed to a recessive male. She produced both hen-feathered and normally feathered sons. The character appears therefore to be sex-limited but not sex-linked. If hen-feathering in the Seabright be rep- resented by S and its normal allelomorph by s, the first cross would be as follows : — Game ? sF s Seabright S S S TTf SsF female Fi Ss hen-feathered male Eggs of 7^1 SF sF S s Sperm of Fi S s F2 Females F2 Males SSF SS hen-feathered SsF Ss hen-feathered sSF sS hen-feathered ssF ss cock-feathered In conclusion, then, in mammals the secondary sexual characters owe their development to the testes. The testes add something to the common inheritance. But in birds the ovary takes something away. THE EFFECTS OF CASTRATION 145 OPERATIONS ON AMPHIBIA The male triton develops each year a peculiar fin or comb on the back and tail. Bresca has found that after castration the comb does not develop. If present at the time of castration, the comb is arrested, but only after several months. Certain color marks pe- culiar to the male are not lost after castration. If the comb is removed in normal males, it regenerates, but less perfectly in castrated males. If a piece of the dorsal fin of the female is transplanted to a normal male in normal position, it may later produce the comb under the influence of the testes. In the frog there appears at the breeding season a thickening of the thumb. Castrated males do not produce this thickening. If it is present in a male at the time of castration it is thrown off, according to Nussbaum, but according to Smith and Shuster its further progress only is arrested. According to Nussbaum and Meisenheimer injection of pieces of testes beneath the skin of a castrated male cause the thumb development to take place, or to continue, but Smith and Shuster question this con- clusion. Such are the remarkable relations that these experi- ments have brought to light. How, we may ask, do the sex glands produce their effect, in the one case to add something, in the other to suppress something? It has often been suggested these glands produce their effects through the nervous system by means of the nerves to or from the reproductive organs. This has been disproved in several cases by cutting the 146 HEREDITY AND SEX nerves and isolating the glands. The results are the same as when they are left intact. This brings us to one of the most interesting chapters of modern physiology, the production and influence of Internal Secretions. INTERNAL SECRETIONS It has become more and more probable that the effects in question are largely brought about by internal se- cretions of the reproductive organs. These secretions are now called ''hormones" or ''exciters." They are produced not only by glands that have ducts or outlets, but by many, perhaps by all, organs of the body. Some of these secretions have been shown to have very re- markable effects. A few instances may be mentioned by way of example. The pituitary body produces a substance that has an important influence on growth. If the pituitary body becomes destroyed in man, a condition called gigan- tism appears. The bones, especially of the hands and feet and jaws, become enlarged. The disease runs a short course, and leads finally to a fatal issue. The thyroid and parathyroid bodies play an im- portant role in the economy of the human body through their internal secretions. Removal leads to death. A diseased condition of the glands is asso- ciated with at least six serious diseases, amongst them cretinism. The thymus secretion is in some way connected with the reproductive organs. Vincent suggests that "the thymus ministers to certain needs of the body before the reproductive organs are fully developed." THE EFFECTS OF CASTRATION 147 Extirpation of the adrenal bodies, another ductless gland, leads to death. Injury to these bodies causes Addison's disease. Finally, the reproductive glands themselves produce internal secretions. In the case of the male mammal it has been shown with great probability that it is the supporting tissues of the glands, and not the germ-cells, that produce the secretion. Likewise, in the case of the ovary, it appears that the follicle cells of the corpus luteum give rise to an important internal secretion. If the sac-like glands are removed, the embryo fails to become attached to the wall of the uterus of the mother. If the ovary itself is removed from a young animal, before corpora lutea are formed, the uterus remains in an infantile condition. From a zoological point of view the recent experi- ments of Gudernatsch are important. He fed young frog tadpoles with fresh thyroid glands. '' They began very soon to change into frogs, but ceased to grow in size. The tadpoles might begin their metamorphosis in a few days after the first application of the thyroid, and weeks before the control animals did so." In contrast to these effects Gudernatsch found that tadpoles fed on thymus grew rapidly and postponed metamorphosis. They might even, in fact, fail to change into frogs and remain permanently in the tad- pole condition. If thyroid, extracts produce dwarfs; thymus extracts make giant tadpoles that never become adults. These examples will suffice to show some of the im- portant effects on growth that these internal secretions may bring about. 148 HEREDITY AND SEX OPERATIONS ON INSECTS The Insects constitute the third great group in which secondary sexual characters are common. The first operations on the reproductive organs were carried out by Oudemans on the gipsy moth, Ocneria {Porthetria) dispar. The male and female are strik- ingly different. Oudemans removed the testes from Fig. 74. — Ovaries of Lymantria (Porthetria) dispar transplanted to male. They have established connection with the sperm ducts. (After Kopec.) young caterpillars and found no change in the color, or size, of the male. He also removed the ovaries from young caterpillars, and again found no effect in the fe- male. The same experiments were later carried out on a large scale by Meisenheimer, who obtained similar results. Meisenheimer went further, however, and per- formed another operation of great interest. He removed the male glands from a male and implanted in their THE EFFECTS OF CASTRATION 149 place the ovary of a female, while it was still in a very immature condition. The caterpillar underwent its usual growth, changed to a chrysaUd, and then to a moth. The moth showed the characters of the male. The presence of the ovary had produced no effect what- ever on the body character of the individual. When this individual was dissected, Meisenheimer found that the ovary had completely developed. It contained mature eggs, and the ovary had often established con- nection with the outlets of the male organs that had i^a B kdb Fig. 75. — Testes of Lymantria {Porthetria) dispar transplanted to female. They have connected with the oviducts. (After Kopec.) been left behind, as seen in Fig. 74, taken from Kopec's description. The converse experiment was also made. The ovaries were removed from young caterpillars, and in their place were implanted the male sex glands from a young male caterpillar. Again no effects were produced on the moth, which showed the characteristic female size and color. On dissection the testes were also found to have grown to full size and to have produced spermatozoa (Fig. 75). These remarkable results, confirmed by Kopec, show 150 HEREDITY AND SEX that in these insects the essential organs of reproduc- tion have no influence on the secondary sexual char- acters of the individual. They show furthermore that the male generative organs will develop as well in the female as in the body of the male itself, and vice versa. It is evident, then, in insects (there is a similar, but less complete, series of experiments on the cricket). Fig. 76. — Papilio Memnon. 1, male; 2, 3, 4, three types of females. (After Meijere.) that the heredity of the secondary sexual characters can be studied quite apart from the influence of the sex glands. How, then, are they inherited so that they appear in one sex and not in the other sex? Within the last two or three years the inheritance of the second- ary sexual differences in insects has been studied. First, there is the case of the clover butterfly, Colias philodice, that Gerould has worked out, where there THE EFFECTS OF CASTRATION 151 are two types of females and one kind of male (Fig. 66). Without giving the analysis of this case I may say that the results can be explained on a Mendehan basis. The peculiar feature of Gerould's explanation is that two doses of the yellow-producing determiner in the female give yellow color — one dose gives white. In the male, on the other hand, one dose of yellow gives yellow. The second case is that of Papilio memnon, worked out by de Meijere from the experiments of Jacobson. There is one male type and three female types, Fig. 76. De Meijere accounts for the results of matings in this species recorded by Jacobson on the assumption of three factors, one for each type of female. The three factors are treated as allelomorphs, and therefore only two of them can be present in any one individual, and since they are allelomorphs they will pass into different gametes. The order of dominance is Achates, Agenor, Laomedon. The male carries these same factors, but they are not effective in him. Baur accounts for the results in a somewhat different way, but involving or- dinary Mendehan conceptions. An interesting case is that reported by Foot and Strobell. They crossed a female of a bug, Euschistus variolarius, the male of which has a black spot on the end of the body (the female lacking the spot), with a male of Euschistus servus th^t lacks the spot both in the males and the females (Fig. 77). The daughters had no spot ; the sons had a faint spot, less developed than in variolarius. When these (Fi) offspring were inbred, they obtained 249 females without a spot. 152 HEREDITY AND SEX 107 males with a spot (developed to different degrees), and 84 males without a spot. The authors give no explanation of their results — but they use the re- FiG. 77. — To left, in 1, is male of Euschistus variolarius, to right male of E. serous. 2 and 3 show eight F2 males ; 4 shows seven F2 males frona another mating. (After Foot and Strobell.) suits to discredit some of the explanations, that rest on the assumption that the chromosomes are the chief factors in Mendelian heredity. I venture, neverthe- less, to suggest the explanation shown on the accom- THE EFFECTS OF CASTRATION 153 panying diagram (Fig. 78) . The analysis rests on the assumption that neither one, nor two doses of S in the female is able to produce a spot, while in the male one dose of S suffices. E. variolarius ? SX ~ SX E. servus S sX — s <^ %\CA F, <. Gametes of /^l sXSX spotless 9 /<:S' cP^"^ ^<^^ sXS spotted S /^- -^ '*^'3^-^> ^<^ ' ^ I Eggs sX — SX ' ' ^ i— U' i Sperm sZ — SX — s — S ■^•^ '>}) -^, F, sXsX sXSX SXsX SXSX sXs sXS SXs SXS <^yN^. > ^ K / spotless ? spotless (? spotted (J spotted (J spotted cJ Fig. 78. — Diagram to show inheritance of spot when E. variolarius (?) is mated to E. servus (d). S ^ spot. s= no spot. X = sex chromosome, that does not carry the factor S for spot. It is very important to understand just what is meant by this ; for otherwise it may seem only like a restate- ment of the facts. In the F2 female with the formulae SXSX, with two doses of the S factor, no spot is as- sumed to appear (nor in the hybrid female >SXsX). At first sight it seems that a female having the formula SXSX is only double the male with sXS, especially if small s is interpreted to mean absence of spots. But this view, in fact, involves a misconception of what the factorial hypothesis is intended to mean. 154 HEREDITY AND SEX To make this clearer, I have written out the case more fully : X A BC S X ABCS 2 X ABC S ABCs S In this, as in all such Mendelian formulae, the result (or character) that a factor produces depends on its relations to other things in the cell (here ABC). We are dealing, then, not with the relation of X to aS alone, but this relation in turn depends on the proportion of both X and S to A B C. It is clear, if this is admitted, that the two formulae above — the one for the male and the other for the female — are neither identical nor multiples. It will be noted that in only one of these attempts to explain in insects the heredity of the secondary sexual characters have the factors for the characters been assumed to be carried by the sex chromosomes. If one accepts the chromosome basis for heredity, these results may be explained on the assumption that the factors lie in other chromosomes than the sex chromo- somes. In the next case, however, that I shall bring forward the factors must be assumed to be in the sex chromosomes themselves. The mutant of drosophila with eosin eyes that arose in my cultures is the case in question. The female has darker eyes than the male. The experimental evidence shows that the factor for eosin is carried by the sex chromosomes. In the female it is present, therefore, in duplex, or, as we say, in two doses ; in the male in one dose. THE EFFECTS OF CASTRATION 155 The difference in color can be shown, in fact, to be due to this quantitative relation. If, for instance, an eosin female is mated to a white-eyed male, her daughters have light eyes exactly like those of the eosin male. The white-eyed fly lacks the eosin factor in his sex chromosomes (as suitable matings show), hence the hybrid female has but one dose of eosin, and in consequence her eye color becomes the same as the male. In this case a sex-linked character is also a secondary sexual character because it is one of the rather unusual cases in which a factor in two doses gives a stronger color than it does in one dose. PARASITIC CASTRATION OF CRUSTACEA Let us turn now to a group in which nature performs an interesting operation. Giard first discovered that when certain male crabs are parasitized by another crustacean, sacculina (a cirriped or barnacle), they develop the secondary sexual characters of the female. Geoffrey Smith has confirmed these results and carried them further in certain re- spects. Smith finds that the spider crab, Inachus mauritanicus, is frequently infected by Sacculina neglecta (Fig. 79). The parasite attaches itself to the crab and sends root-like outgrowths into its future host. These roots grow like a tumor, and send ramifications to all parts of the body of the crab. The chief effect of the parasite is to cause complete or partial atrophy of the reproductive organs of the crab, and also to change the secondary sexual charac- ters. Smith says that of 1000 crabs infected by 156 HEREDITY AND SEX sacculina, 70% of both males and females showed alterations in their secondary sexual characters. As a control, 5000 individuals not infected were ex- amined, only one was unusual, and this one was a her- maphrodite (or else a crab recovered from its parasite). Fig. 79. — A mala of Inachus mauritanicus (upper left hand). Female of Inachus scorpi (lower left hand). Male of Inachus mauritanicus carrying on its abdomen two specimens of Danalia curvata and a small Sacculina neglecta (upper right hand). Male of Inachus mauritanicus with a Sacculina neglecta on it (lower right hand). The abdomen and chelae of the host are inter- mediate in character between those of an ordinary male and female. (After Geoffrey Smith.) As the figures (Fig. 80) show, the adult male has large claws ; the female, small ones. He has a narrow abdomen ; she has a broad one. In the male there is a pair of stylets on the first abdominal ring (and a pair of greatly reduced appendages behind them). The adult female has four biramous abdominal append- ages with hairs to carry the eggs. THE EFFECTS OF CASTRATION 157 8. 11. 7 9 iO. 12 Fig. 80. — 1, adult normal male; 2, under side of abdomen of normal 158 HEREDITY AND SEX The infected males '^show every degree of modi- fication towards the female type." The legs are small, the abdomen broad, the stylets reduced, and the typical biramous appendages with hairs appear. When the female crab is infected she does not change ''toward" the male type, although the ovary may be destroyed. The only external change is that the abdominal appendage may be reduced. In a hermit crab, Eupagurus meticulosus , infected by Peltogaster curvatus, similar results have been obtained. The infected male assumed the ordinary sexual char- acters of the female, but the females showed no change towards the male. In these cases it seems probable that the testes of the male suppress the development of the secondary sexual characters that appear ordinarily only in the females. The case is the reverse of that of the birds and different again from that of the mammals. In birds and mammals the secondary sexual charac- ters are in many cases directly dependent on the in- ternal secretions of the sex glands. These secretions are carried alike to all parts of the body, hence the absence of bilateral gynandromorphs in these groups. adult male; 3, male infected with sacculina, showing reduction of chela and slight broadening of abdomen; 4, 5, showing attenuated copulatory styles and slight hollowing out of abdomen; 6, under side of abdomen of a similar male specimen, showing reduction of copulatory styles and presence of asymmetrically placed swimmerets characteristic of female; 7, infected male which has assumed complete female appearance; 8, under side of abdomen of 7, showing reduced copulatory styles and swimmerets; 9, under side of abdomen of similar male specimen with well-developed copulatory styles and swimmerets; 10, adult female, normal; 11, under side of abdomen of 10, showing swimmerets and trough-shaped abdomen; 12, under side of abdomen of infected female, showing reduction of swimmerets; 13, immature female showing small fiat abdomen; 14, under side of abdomen of 13, showing flat surface and rod-like swimmerets. (After Geoffrey Smith.) THE EFFECTS OF CASTRATION 159 CONCLUSIONS In conclusion it is evident that the secondary sexual characters in four great groups, viz. mammals, birds, Crustacea, and insects, are not on the same footing. Their development depends on a different relation to the reproductive organs in three of the groups, and is independent of the reproductive organs in the fourth. It is not likely, therefore, that their evolution can be explained by any one theory, even by one so broad in its scope as that of sexual selection. If, for example, in the mammals a more vigorous male, due to greater development of the testes, were ''selected" by a female, the chances are that his second- ary sexual characters will be better developed than are those of less vigorous males, but he is selected, not on this account, but because of his vigor. If a male bird were ''selected" on account of greater vigor, it does not appear that his secondary sexual characters would be more excessively developed than, those of less vig- orous males, provided that his vigor were due to the early or greater development of the testes. If in birds the male by selecting the female has brought about the suppression of the male plumage, which is their common inheritance, he must have done so by selecting those females whose ovaries produced the greatest amount of internal secretions which suppresses male-feathering. Moreover, he must have selected, not fluctuating variations, but germinal variations. In insects the development of the secondary sexual characters is not connected with the condition of the reproductive organs, but is determined by the complex 160 HEREDITY AND SEX of factors that determines sex itself. If selection acts here, it must act directly on germinal variations, that are independent in origin of the sex-determining factor, but dependent on it for their expansion or suppression. These considerations make many of the earlier state- ments appear crude and unconvincing ; for, they show that the origin of the secondary sexual characters is a much more complex affair than was formerly im- agined. These same considerations do not show, however, that if a new germinal character appeared that gave its possessor some advantage either by accelerating the opposite sex to quicker mating or by being corre- lated with greater vigor and thereby making more certain the discovery of a mate, such a character would not have a better chance of perpetuation. But in such a case, the emphasis no longer lies on the idea of selection with its emotional imphcations, but rather on the appearance of a more effective machine that has arisen, not because of selection, but, having arisen quite apart from any selective process, has found itself more efficient. Selection has always implied the idea that it creates something. Now that the evidence indicates that selection is not a guaranteed method of creating anything, its efficiency as a means of easy explanation is seriously impaired. CHAPTER VI Gynandromorphism, Hermaphroditism, Parthenogenesis, and Sex Three different sex conditions occur in animals and plants that have a direct bearing on problems of Heredity and Sex. • The first condition is called Gynandromorphism — a condition in which one part of the body is like the male, and the other part like the female. 1 4 The second condition is called Hermaphroditism — a condition in which the individuals of a species are all alike — maleness and femaleness are combined in the same body. Two sets of reproductive organs are present. The third condition is called Parthenogenesis — a condition in which the eggs of an animal or plant develop without being fertilized. gynandromorphism Gynandromorphs occur most frequently, in fact almost exclusively, in insects, where more than one thousand such individuals have been recorded. They are most abundant in butterflies, common in bees (Fig. 81) and ants, rarer in other groups. They occur relatively more often, when two varieties, or species, are crossed, and this fact in itself is signifi- cant. A few examples will bring the cases before us. In my cultures of fruit flies several gynandro- 161 162 HEREDITY AND SEX morphs have arisen, of which two examples are shown in Fig. 82. In the first case the fly is female on one side, as shown by the bands of her abdomen, and male on the other side (upper right-hand drawing). In the second case the fly looked like a female seen from above. But beneath, at the posterior end, the genital organs of the male are present, and normal Fig. 81. — A gynandromorph mutillid wasp, Pseudomethoca canadensis, male on right side, female on left side. (After Wheeler.) in structure. In the latter case the fly is ostensibly a female, except for the male organs of reproduction. How can we interpret these cases? We find a clue, I think, in the bee. It is known that if the egg of the bee is fertilized, it produces a female — only female-producing sperms are formed. If it is un- fertilized, it produces a male. In the bee two polar bodies are produced, and after their extrusion the num- ber of chromosomes is reduced to half, as in ordinary cases. The haploid number produces a male ; the double number produces a female. Boveri pointed out that if through any chance the GYNANDROMORPHISM 163 entering sperm should fail to reach the egg nucleus before it divides, it may then fuse with one of the halves of the egg nucleus after that divides. From the Fig. 82. — Two gynandromorphs of Drosophila ampelophila. Upper left-hand figure, female dorsally, male ventrally (as seen in third figure, lower line). Upper right-hand figure, male on left side, female on right, and correspondingly the under side shows the same difference (lower row, last figure to right. Lower row from left to right; normal female, normal male, vertical gynandromorph and lateral gynandromorph. half of the egg containing the double nuclei female structures will develop ; from the other half, contain- ing the half number of chromosomes, male structures (Fig. 83, A). Here we have a very simple explanation of the gynandromorphism. 164 HEREDITY AND SEX There is another way in which we may imagine that the results are brought about. It is known that two or Fig. 83. — Diagram, illustrating on left (A) Boveri's hypothesis, on right (B) the author's hypothesis, of gynandromorphism. more spermatozoa frequently enter the egg of the bee. Should only one of them unite with the egg nucleus, the parts that descend from this union will be female. If any of the outlying sperm should also develop, i GYNANDROMORPHISM 165 t":^ T^^'' '"PP°^^d t-^ P'-oduce male structures (Fig. 83, B). The first case of the fly, in which one half the body IS male and the other female, would seem better in accord with Boveri's hypothesis. In its support also may be urged the fact that Boveri and Herbst have shown that the belated sperm-nucleus may unite with one of the two nuclei that result from the nrst division of the egg nucleus. On the other hand, the second case of the fly (where only a small part of the body is male) may be better accounted for by my hypothesis. It is known that single sperms that enter an egg without a nucleus or even with one, may divide. The two hypotheses are not mutually exclusive, but rather supplementary. ; loyama has described a gynandromorph in the silkworm that arose in a cross between a race with a banded caterpillar (the female parent) and a race with a white caterpillar (the male parent). As shown m Fig. 84, the gynandromorph was banded on the left (maternal) side and white on the other (right) side When the adult moth emerged, the left side was female and right side was male. Since the sperm alone bore the white character, which is a recessive character it appears that the right side must have come from sperm alone. This is in accordance with my hypothesis, but 1 have also shown that Gynandromorphs may arise through somatic dislocations of the sex chromosomes m the early embryo. Gynandromorphs are not un- common in insects, rare (or never present) in birds and mammals. The explanation of this difference is found, I think, in 166 HEREDITY AND SEX (t. % I'ltr. I -^^^ /); // /vi, ///. • ^=S«y ^ /■lA'- in. . :•: ic io •:•> 20 10 y ^.'i/>e//ii Fig. 91. — Illustrating chromosome cycle in Neuroterus. A, one type of spring female, whose eggs (containing 20 chromosomes) produce no polar bodies. Only sexual females result. B, the other type of spring female whose eggs form two polar bodies, leaving 10 chromosomes in egg. These eggs give rise to males. C, ripening of egg of sexual female (2d generation), and D, spermatogenesis of male (second generation). These eggs produce sexual females (in left-hand side of Fig. 91). From the eggs of other parthenogenetic fe- males two polar bodies are given off, and the half (10) number of chromosomes is left in the egg (see right-hand side of Fig. 91). These eggs produce males. The life 178 HEREDITY AND SEX cycle finds its explanation in these relations except that the origin of the two kinds of parthenogenetic females is unexplained. If we were justified in assuming that two classes of female-producing sperm are made in the male, even this point would be cleared up, for in this ^// '-^^^^jw'^ 7j///{ .x"f vr/ (■^/'f//urY7///a .^'///f/ ^/fff/nr Wi Fig. 92. — Life cycle of Phylloxera carycecaulis. way the two classes of parthenogenetic females could be explained. In another group of insects, the aphids and phyllox- erans, the situation is different. In the phylloxerans of the hickories there emerges in the spring, from a fertilized egg, a female known as the stem mother (Fig. 92). She pierces a young leaf PARTHENOGENESIS 179 with her proboscis, which causes a prohferation of the cells of the leaf. Eventually the leaf cells grow so fast that the stem mother is overarched in the gall that she has called forth. Inside the gall she begins to lay her eggs. From these eggs emerge young individuals that remain in the gall until they pass their last molt, when they become winged migrants. Externally all the migrants are alike; but if they are dissected, it will be found that some of them have large eggs, some small eggs. But all the offspring of the same mother are of one or of the other sort. The migrants crawl out of the opening in the gall and fly away. Alighting on other hickories, they quickly deposit their eggs. From the large eggs the sexual females emerge. They never grow any bigger than the egg from which they hatched. In fact, they have no means of feeding, and contain only one large egg with a thick coat — an egg almost as large as the female herself. From the small eggs of the migrants, minute males are produced — ripe at their birth. They fertilize the sexual female. She then deposits her single egg on the bark of the hickory tree. From this egg (that lies dormant throughout the entire summer and following winter) there emerges next spring a female, the stem mother of a new line. Here we find three generations in the cycle — two of which reproduce by parthenogenesis. The first parthenogenetic generation gives rise to two kinds of individuals — one makes large eggs, the other small eggs. The large eggs produce sexual females, the small eggs males. 180 HEREDITY AND SEX A study of the chromosomes has explained how some of these changes in successive generations are brought about. It has explained, for instance, how males are produced by parthenogenesis, and why the sexual egg produces only females. Let us take up the last point first. When the spermatocytes are produced, we find, as in many other insects, that at one division a sex chromo- some passes to one cell only (Fig. 93). Two classes of cells are produced — one with three, one with two, chromosomes. The latter degenerates, and in conse- quence only the female-producing spermatozoa become functional. All fertilized eggs give rise therefore to females. The second point that has been made out concerns the production of the male. When the small egg produces its single polar body, all of the chromosomes divide, except one, which passes out entire into the polar body. In consequence the number of chromo- somes left in the egg is one less than the total number. In a word, there are five chromosomes in the male, while there are six chromosomes in the female (Fig. 93). By throwing out one chromosome, the change is effected. The chromosome is the mate of the sex chromosome, that appeared as a lagging chromosome in the spermato- genesis. In the large egg no such diminution takes place, consequently the diploid number of chromosomes is present in the female. These unite in pairs and are reduced to three when the two polar bodies of the sexual egg are produced. We see that by means of the chromosomes we can PARTHENOGENESIS 181 bring this case into line with the rest of our informa- tion bearing on the relation of the chromosomes to sex. One important point still remains to be explained.* What causes some of the migrants to produce large I'HYLLOXERA CAnYMCAULTfi T'o^ev^ T»^cae^ A ^ r^ O^nt^v of ffZn^efi -^en. I C:::d •^n? o o o o o o \ •^^ocfM/ fs-ma/t. ^p ^' .^ X is X 4. o 0 7vCa/f S^iiyndlc <3 ^f \ ^- TKaZe' ■J€CtmA Fig. 93. — Chromosomal cycle of P. carycecauUs. 182 HEREDITY AND SEX eggs and others small eggs ? There must be either two kinds of stem mothers or one kind with double po- tentiahty. Inasmuch as in other parthenogenetic types there is experimental evidence to prove that environ- mental conditions determine which alternative state, whether male-producing or female-producing individ- ual, is realized, so here we may, provisionally, follow the same interpretation. Once the course is deter- mined the subsequent internal events follow for two generations in a definite order. If the stem mother has been affected in one way, all of her daughters produce large eggs ; if in the other way, small eggs. 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- tiUzed 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. PARTHEXOGENESIS 183 J^icrt&f / ^ ^ y S' 9 yff .// /^ ^s /^ y^ Fig. 94. — Life cycle of Simocephalus ; successive broods in horizontal lines, successive generations in vertical lines. (After PapanlcolaJ!) 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 suggests, 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 1 §5 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. More recently Grosvenor and Smith have found for Moma that if females are reared alone (at 25°-30° C ) no sexual broods appear, while parallel cultures of females crowded together give 30 or higher per cents of males. Agar has carried isolated females through 46 parthenogenetic generations and has found that mothers from late brood also give only parthenogenetic offspring m a suitable environment. A third type, Hydatma senta (Fig. 95), an almost microscopic wormlike animal belonging to the rotifers reproduces by parthenogenesis. The resting egg always gives rise to a parthenogenetic female, i,e. she 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 hke 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 spermatozoon 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 fertihze 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 Hydat/na senta 9 7?. ^M. 9 '< -f/t/l/ V 7/^il/e: - t^^ /^fi^//fH . ^/M< Fig. 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 eggs 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 zoologist, 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 Spring Water Old Cttlture Filtrate One-fourth One-half Three-fourths Undihited d" 2 ? ? d ? ? ? d ? 9 ? cf 9 ? 9 d 9 9 9 26 177 25 407 15 350 8 362 0 337 %oid9 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. Mitchell found that a sudden change in food causes male-producing individuals to appear in Asplanchna; and Whitney can now produce, at will, in Hydatina a high percentage of male producers. Females feeding on the colorless flagellate Polytoma were fed on the green flagellate Duniella, and gave birth to 80 % of male producers ; while the control female fed on Poly- toma gave birth to only 9 % of male producers. Since the eggs of the male producer give rise to sexual fe- males, or to males, according to whether they are fer- tilized or whether they are not, sex itself is not here determined by the environment, but by fertilization ; the environment determines the kind of reproduction. 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 in 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 fertiUzed. We have seen that such eggs without being fertihzed 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 lies, 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 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 unfertihzed 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. Loeb has shown by a thorough study of the conditions that any substance that causes cytolysis (a typical form of cell destruction) will induce parthenogenesis if ap- plied to the surface of the egg only. 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 cc. 192 HEREDITY AND SEX of sea water plus 8 cc. of 23^ 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 role in fertihzation. 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 physiological point of view. In addition, we have seen that fertihzation 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 animals 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 males. 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 a male. In the case of the bee the formula is XABC, which also gives a male. All else is hypothetical and premature, but if these two formulae 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. |>^ i ^ ^^ R A R Y I ^ 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 have 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. jp TP 7? J? JP J? J? Pi r2 rs Pi Ts Pq — Pu 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 case 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 is sim- ilar to self-fertilization in hermaphroditic forms is not known. Darwin gives results of self-fertilization in Ipo- moea 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. ShuU 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 paramoecium 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 paramoecia on FERTILITY 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. Recently Woodruff has found in his long-lived race that under proper conditions individuals will conjugate. Woodruff and Erdmann found in this race, although not allowed to conjugate, that periodically the macro- nucleus breaks down and several micromere divisions take place. Finally a new nuclear apparatus of micro- nuclear origin is reconstructed. The process is com- parable to the nuclear changes prior to conjugation except that the last micronuclear division is omitted. This periodic change is not peculiar to this race of Paramoecium, but appears in other races that regularly conjugate. 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 is 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, ^ The upper line Fi-Fis gives the average output of flies per pair. Below this line the percentages mean the number of isolated eggs that hatched. 200 HEREDITY AND SEX 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. The egg counts show that in the inbred stock many of the eggs are not fertiHzed, or if fertihzed (32%) 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. M\sTovy oF lnt>recl Stock. Fl a 3 4 5 6 7 368 209 i9f m es m - 8 9 10 11 iZ R5 - - - - - 156 Cro66 of fia i^^-utxc^tc 52% 58% Fig. 96. — The horizontal line Fi-Fn 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 F13. 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 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 fertiUzes the pistil of another long-styled plant the vigor of the offspring is less than when the same kind of pollen is used to fertiUze the pistil of a short-styled flower. The next table gives the detailed results. Nature of Union Number of Flowers Fertilized Number of Seed Capsules Maximum OF Seeds in Any One Capsule Minimum 3F Seeds in Any One Capsule Average No. of Seeds per Capsule Long-styled form by pollen of short- styled form: Legitimate union. 10 6 62 34 46.5 Long-styled form by own-form pollen: Illegitimate union. 20 4 49 2 27.7 Short-styled form by pollen of long- styled form : Legitimate union. 10 8 61 . 37 47.7 Short-styled form by own-form pollen : Illegitimate union. 17 3 19 9 12.1 The two legitimate unions together. 20 14 62 34 47.1 The two illegitimate unions together. 37 > 49 2 21.0 We know now that these two types of plants — long- styled and short-styled — differ from each other by a single Mendehan 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. Fig. 97. — At left of figures there are two strains of pure bred corn and 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. ShuU 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. East and Shull find that when two strains of corn FERTILITY 203 (that have been to a large extent made pure) are crossed, the offspring is more vigorous than either parent (Fig. YiG. 98. — At left an ear of Leamuig 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 pic- 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 Fi corn is self-fertiUzed, it produces a very mixed progeny, more variable than itself. Some of the F2 offspring are like the original grandparental strains, some like the corn of first generation, and others are intermediate (Fig. 99). No a, i/t. 6 bu pffocrc %inS buetretrt «t Fig. 99. — No. 9 and No. 12, two inbred strains of Learning Dent corn compared with Fi and Fi (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 Fi 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 Vigor, as Measured by Various Char- acters, IN Six Successively Inbred Parthenogenetic Lines OF Hydatina senta g 2 02 I. II. Character to be Measured Size of family of parthenogenetic female . . Size of family of fertilized sexual female . . Number of eggs laid per day Number of days required to reach maturity Proportion of cases in which first daughter did not become parent Same in percentages Size of family of parthenogenetic female . . Size of family of fertilized sexual female . . Number of eggs laid per day Number of days required to reach maturity Proportion of cases in which first daughter did not become parent Same in percentages Number of Parthenogenetic Line 1 2 3 4 5 6 48.4 42.5 46.8 42.5 31.0 22.6 16.7 12.8 12.8 11.5 6.3 7.3 11.0 11.4 10.3 10.0 9.2 7.5 2.27 1.66 2.25 1.93 2.25 2.12 1/11 1/3 2/4 3/16 0/4 5/8 14.2 25.0 41.6 48.4 30.8 41.0 37.0 33.8 24.8 16.7 13.7 13.5 15.2 10.1 7.6 11.0 11.6 7.9 7.7 9.6 8.6 2.27 1.55 2.57 2.20 1.90 2.00 1/11 4/9 2/7 2/10 8/20 7/16 21 ..0 23.5 41.6 number of times the first daughter was too weak to become the mother of a new hne. It is clear that inbreeding did not lead to an increase in vigor. In paramoecium 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 paramoe- cium. Two individuals are represented by black and white circles. At the time of conjugation the small or micronucleus in each divides (5), each then divides again (C). Four nuclei are produced. One of these micronuclei, the one that lies 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-con jugants are formed. Each has a new double nucleus. This nu- 208 HEREDITY AND SEX cleus divides (G) and each daughter nucleus divides again (H), so that each ex-con jugant has four nuclei. Fig. 100. — Diagram to show the history of the micronuclei of two Paramoecia during (A-F) and after (F-J) conjugation. Compare this dia- gram with Fig. 2. Another division gives eight nuclei in each. The para- mcecium 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- con jugant four new paramoecia are produced, which now proceed to divide in the ordinary way, i.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 {E), 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, i.e. homozygous, in many factors, and different, i.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-con jugants are more like each other than were the original strains. Calkins has obtained som^e new and important facts concerning the likeness and unlikeness of the new strains that result from conjugation. He has used wild, i.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-con jugants is shown in the next diagram (Fig. 101). The four Unes, ''quadrants/' (1, 2, 3, 4) that are descended from each of six ex- conjugants (viz. (r, H, L, M, Q, B) are shown. At intervals large numbers of the populations were put under conditions favorable to conjugation and the -^on^ti^aiicTv t^atia/ifTt^ i^ @fc» €ae-am/if^ian£t cf t^a^uz/neeilim ccui^atum/ /3 /6 7/ ,/aM\Ja X <: O ifi 7*rf maiireir ofil 4^ — 46m -fO /9/4 'J/v S* — «7>* 2i5W rrtr AtSerifs J>e c. ZV» P'a /i to tj ^an < < /^ /V •<7 Test Fei &- -ra 1 Pat 'V Test] Jnor. S.9 -aAf Z9 /8 0Z Tett Opl Jejr ^: 2J < i* <5 -»* -m ^G- /6'i6 //SerUs () ^ c> -Ht- z\o -J.«- -tiit Ho -m- -*y -»*- -f.»- t^- StrieJ 7i X Z9 JSt Jenes "■Aa () **- 0 -J-% < i f? 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 ; X 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-con jugant 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 paramcecium 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 (F2) 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 (Fi), and therefore hke it, most of them will be simpler 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 decUne does not take place. An alternative view, that is also MendeUan, 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 fertihty 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 Li, is a non-sex-linked char- acter. If it 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 Li and L2 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, L2, is sex-hnked, 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 latteriraacc of fcrUWfy in fowl. C^earl) Lovv9 F. L, L, fAz — L^ Lotv9 iXcrocf) I, -61 -Iz — ^2. (Z^r^^ r. L, l^ L, l^ Low 9 Lai, L^i, (to^) cf f,L^l^L^l^ 4(i^K9 9 F— L^ c5 Z^. "^2 -Hi^k 9 (Low) C? Lo*v 9 T-^^ Lw9 -^i-fj; (Lo4^ Fig. 102. — Illustrating Pearl's hypothesis. F = female factor present in half of the eggs and determining sex. Li = factor for low egg produc- tion; li, its allelomorph for zero production of winter eggs. L2 = factor for high winter production; h, 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 cases where lowered fertility was due to eggs failing 1 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 pratensis. 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 hybrid 9 B ^ (3 r %• B 4- 4tifG ■f- -i- 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 h. 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 -t- signs indicate successful fertihzation. Correns' theory is also in accord with other com- FERTILITY 217 binations that he made. There can be httle 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. S. Adkins. Five individuals are here used. The eggs of one individual, A, were placed in five dishes (horizontal line) ; likewise those of B, C, D, E. The sperm of A, designated by a (vertical lines) was used to fertilize the eggs. A, B, C, D, E ; likewise the sperm h, 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, i.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 j^etf and Cross MrIi7/za7jo/i /Jn Oor7a. A" o A^ A° 9Z 8^ A^ &6 B" 6s B^ o 35 9^ B" •9/ 9^ 96 O 97 9e 98 o ; R.<0>-5i>]CX XX X( 9 9 cr a Fig. 115. — Diagram to indicate heredity of color bUndness through female. A color-blind female transmits color blindness to all of her sons, to half of her granddaughters and to half of her grandsons. 242 HEREDITY AND SEX substitute white eyes for haemophilia, the scheme already given for white versus red eyes in flies applies to this case. If, for instance, the mother with normal eyes has two X chromosomes (Fig. 114), and the fac- tor for haemophilia is carried by the single X in the male (black X of diagram), the daughter will have one affected X (and in consequence will transmit the factor), but also one normal X which gives normal CnU OiU o Orn 6 6 6 6 6 Fig. 116. — Pedigree of Ichthyosis from Bramwell. (After Davenport.) vision. The sons will all be normal, since they get the X chromosomes from their mother. In the next generation, as shown in the diagram (third line), four classes arise, normal females, hybrid females, normal males, and hsemophilious males. Color blindness fol- lows the same scheme, as the above diagrams illustrate (Figs. 114 and 115). In the first diagram the color- blind male is represented by a black eye ; the normal female by an eye without color. The offspring from SPECIAL CASES OF SEX-INHERITANCE 243 r^ -^ -^ ^ ^ -a -J ^ 04 CO c; -a -D £3 (-0 '—a — o -z -z -z o P 0) e3 M a o (-1 s 03 & o 03 I 03 —2 2 c3 u 3 r-O g ^ I J— O "3 — O 4, M « 244 HEREDITY AND SEX two such individuals are normal, but the color blindness reappears in one-fourth of the grandchildren, and in these only in the males. The reverse mating is shown in the next diagram in which the female is color-blind. She will have color-blind sons and normal daughters (criss-cross inheritance), and all four kinds of grand- children. Other cases in man that are said to show sex-linked inheritance are atrophy of the optic nerve, multiple r L D iSi "lo »iO ^ o it Fig. 118. — Pedigree of night blindness in a negro family, from Bordley. (After Davenport.) sclerosis, myopia, ichthyosis (Fig. 116), muscular atrophy (Fig. 117). Night blindness is described in certain cases as sex-hnked ; in other cases, however, a disease by the same name appears to be a simple domi- nant and not sex-hnked (Fig. 118). All these cases of sex-linked inheritance in man are explained by the assumption that the factor that produces these characters is carried by the sex chromo- some, which is duplex (XX) in the female and simplex (X) in the male. A simpler assumption has not yet been found. If one is fastidious and objects to the SPECIAL CASES OF SEX-INHERITANCE 245 statement of factors being carried by chromosomes, he has only to say, that if the factors for the characters follow the known distribution of the sex chromosome, the results can be accounted for. The culmination of the evidence of sex-determina- tion in man is found in a study of the cell structure of the human race itself. Strange as it may seem, we have been longer in doubt concerning the number of chromosomes in man than in any other animal as extensively studied. Four conditions are responsible : (1) The large number of chromosomes present in man. (2) The clumping or sticking together of the chromo- somes. (3) The difficulty of obtaining fresh material. (4) The possibility that the negro race has half as many chromosomes as the white race. Two years ago Guyer announced the discovery that in all probabihty there exist in man two unpaired chromosomes in the male (Fig. 119) that behave in all respects like that in the typical cases of the sort in insects, where, as we have seen, there are two classes of spermatozoa, differing by the addition of one more chromosome in one class. These produce females ; the lacking class produces males. But Guyer's evidence was not convincing. He found in all 12 chromosomes in one class of sperm and 10 in the other. Mont- gomery has also studied the same problem, but his account, while confirming the number, is in disagree- ment in regard to the accessory. Jordan has gone over a number of other mammals, in some of which he thinks that he has found indica- tions at least of two classes of sperm. Still more recently another investigator, von Wini- 246 HEREDITY AND SEX warter, has attacked the problem (Fig. 120). His material and his methods appear to have been superior to those of his predecessors. His results, while stated with caution and reserve, seem to put the whole question on a safer basis. His main results are illustrated in the diagram 3 i^^^^^'"^ M Fig. 119. — Human spermatogenesis according to Guyer. The sex chromosomes are seen in 6-9. (Fig. 120). In the male he finds 47 chromosomes. Of these 46 unite at reduction to give 23 double chromosomes — one remains without a mate. At the first reduction division the pairs separate, 23 going to each pole, the unpaired chromosome into one cell only. SPECIAL CASES OF SEX-INHERITANCE 247 At the next division all the chromosomes in the 23 group divide, likewise all in the 24 group divide. There are produced two spermatozoa containing 24 ^ 5/ 4»V Xe F ^fV* *l^> ' ? h. 1 i '^?. g^ J k f tu ^fltttt— . < Fig. 120. — Human spermatogenesis according to von Winiwarter, a, spermatogonia! cell with duplex number; h, 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 ; i, first division completed ; j, second spermatocyte with 23 chromo- somes ; k, second spermatocyte ^ath 24 chromosomes ; I, 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 ySi-.x- <(efer'/ni /u/Ucn f/i ///an (If/zifmr tier ) hyp:- A 11 o (jCt '^°* ^ iV):' ,^u-..M., .3 25tX B J 3 - ^ '• : :'^ f^^. 23' t5 D Fig. 121. — Diagram of human spermatogenesis. A, spermatogonia! 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 ; E, 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. If these observations are confirmed, they show that in man, as in so many other animals, an internal mechanism exists by which sex is determined. It is futile then to search for environmental changes that SPECIAL CASES OF SEX-INHERITANCE 249 might determine sex. 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INDEX Abraxas, 128 Achates, 151 Achia, 106 Addison's disease, 147 Adkins, 217-218 Adrenal, 147 Agenor, 151 Allen, 113 Amphibia, 145 Amphipoda, 117 Andrews, 117 Angiostomum, 170 Antlers, 110, 133 Ants, 117 Argentine, 227 Argonauta, 26 Aristotle, 35 Armadillo, 238 Ascaris, 20, 21, 49 Ascidian, 217 Baltzer, 55, 58, 61 Bancroft, 194 Barnacle, 155 Bateson, 72, 75, 99, 100, 125 Baur, E., 99 Beans, 123 Bee, 174, 175, 176, 220 Bees, 32 Beetles, 106 Bell, 232 Belt, 102 Bird of paradise, king, 109 six-shafted, 109 superb, 109 Black, 96-97 Blakeslee, 171 Bobolink, 27 Boring, 51 Boveri, 51, 55, 58, 162, 165, 171 Bresca, 145 Bridges, 223, 224 Bruce, 212 Bryonia, 171-172 Butschli, 8 Calkins, 8, 198, 206, 209, 210 Callosamia, 116 Capons, 142, 143 Cardamine, 215-216 Castle, 195 Ceylon, 125, 127 Checker diagram, 78 Chemotaxis, 117 Chidester, 117 Cicada, 106 Ciona, 217-218 Clipped wings, 119 Colias, 129-130, 150 Collins, 202 Color-blind, 241 Color blindness, 242 Conger eel, 2 Corpus luteum, 147 Correns, 74, 79, 99, 171, 172, 215. 216 Crab, 155 Cretinism, 146 Cricket, 150 Criodrilus, 168 Cuenot, 232, 233 Cunningham, 121 Daphnians, 182-185, 189 Darwin, C, 73-74, 101, 103, 104, 107, 112-114, 120, 125, 142, 194, 197, 200-202 Davenport, C, 72, 143, 239 Deer, 110, 133, 134 Delage, 193 Dinophilus, 234 Diplogaster, 225 170, Doncaster, 176 Dorsets, 134, 135, 136, 137, 138 Drelincourt, 232 Drone, 175 281 282 INDEX DrosophUa, 63-68, 96, 117, 130 Diising, 233 East, 99, 202, 204, 211 Edwards, 51 Egret, 111 Eland, 136 Elaphomyia, 106 Elephant, 110 Emerson, 99 Eosin eye, 130, 154, 155 Eupaguras, 158 Euschistus, 151 Fabre, 220, 221 Fielde, 117 Firefly, 28, 30, 31 Fishes, 32 Florisiiga, 102 Foot, 151 Forel, 117 Frog, 145, 147, 228 Frolowa, 51 Fruit fly, 117, 195, 196, 221 Fundulus, 32 Gall, 179 Galton, 236 Game, 144, 212 Geddes, 232 Gentry, 232 Germ-cells, 23 Gerould, 130, 150, 151 Giard, 155 Gigantism, 146 Goldschmidt, 124 Goodale, 72, 142 Gosse, 103 Growth, 3 Gudernatsch, 147 Guinea hen, 225 Gulick, 51 Guyer, 225-226, 245 Gynandromorphism, 161 Gypsy moths, 117 Habrocestum, 107 Hemophilia, 239, 240, 242 Hectocotylized arm, 26 Henking, 50 Herbst, 55, 61, 62 Herdwicks, 134-135 Hermaphroditism, 161 Hertwig, R., 9, 228, 234 Holmes, 117 Hormones, 146 Horns, 133-138 Hudson, 114, 115 Humming-birds, 103, 108 Hydatina, 2, 185 Hyde, 196, 199, 215 Ichthyosis, 242 Identical twins, 236-239 Inachus, 155 Ipomoea, 197 Italian, 227 Jacobson, 151 Janda, 168 Janssens, 94 Jehring, 238 Jennings, 9, 12, 206-208 Johannsen, 122-125 Jordan, H. E., 245 Keeble, 212 Kellogg, 117, 232 King, 229, 234 Kopec, 149 Kruger, 225 Kuschakewitsch, 228 Lamarckian school, 17 Landois, 232 Langshan, 69-71 Laomedon, 151 Lethal factor, 221-223 Linkage, 93 Lion, 27 Lister, 34 Loeb, J., 62, 190, 191, 192, 193 Lutz, 118 Lychnis, 172-173 Lygseus, 44 Lymantria, 148 McClung, 50 Mffivia, 108 Mallard, 28, 142 Malsen, von, 234, 235, 236 Mammals, 159 Mammary glands, 140 Man, 34, 229, 236-249 INDEX 283 Marchals, 171 Mast, 30 Maupas, 5, 8, 187, 198, 234 Mayer, 116 de Meijere, 151 Meisenheimer, 145, 148-149 Mendel, 84, 73-75, 80, 84 Menge, 34 Merino, 134, 135 Miastor, 21, 174 Mice, 233 Mimicry, 127-130 Miniature wings, 66-67 Mirabilis, 79-80 Moenkhaus, 196 Montgomery, 34, 50, 115, 117, 245 Mosquito, 51 Mosses, 171 Mulsow, 51 Myopia, 242 Nematode, 224-226 Nereis, 36 Neuroterus, 176-177 Newmann, 238 Night blindness, 242 Non-disjunction, 223-224 Nussbaum, 16, 145 Ocneria, 148 Octopus, 25 Oncopeltus, 46, 84 Optic nerve atrophy, 244 Oudemans, 148 Ovariotomy, 135 Owl, HI Papanicolau, 183-185 PapHio, 125-129, 151 Paramoecium, 5, 6, 12, 206-211 Parathyroid, 146 Parthenogenesis, 161 Patterson, 239 Paulmier, 50 Pea, edible, 75-78, 85-88 Pearl, R., 72, 212-213, 227 Pearse, 117 Peckham, 115-116, 120 Pellew, 212 Peltogaster, 158 Petrunkewitsch, 117 Phalarope, 112 Pheasants, 225 Phidippus, 34 Photinus, 28 Phylloxerans, 52, 54, 178, 179, 180, 181, 189 Pigeons, 32 Pituitary body, 146 Plutei, 60 Plymouth rock, 69-71, 212 Polar bodies, 37 Polytmus, 103 Porter, 117 Porthetria, 117, 148 Primula, 201, 202 Promethea, 116 Protenor, 40 Punnett, 127, 128, 138, 233 Rawls, 221 Rat, 140, 233 Reduplication, 100 Reindeer, 136 Rhabditis, 169, 224, 226 Riley, 232 Ritzema-Bos, 195 Rotifers, 185-189 Rudimentary wing, 214, 215 Russo, 234 Sacculina, 155 Sagitta, 21, 22 Schenk, 233 Schleip, 170, 171 Schultze, 233 Sclerosis, 242 Seabright, 143-144 Sea cow, 27 Sea-lion, Steller's, 110 Sea-urchin, 56-62 Segregation, 81, 100 Sex, 83, 84 Sex chromosome, 50, 80, 83, 84 Sex determination, 84 Sex-limited, 83 Sex-linked, 81, 83, 84, 132 Sheep, 134-138 Shull, A. F., 187, 197, 205 Shull, G. H., 173, 202, 204, 211 Shuster, 145 Siamese twins, 236 Silkworm, 117, 165 Sinety, 50 284 INDEX Skeleton, rat, 140 Smith, G., 145, 155 Soule, 116 Sparrow, 2 Spermatophores, 25 Sphaerechinus, 59-60 Spiders, 34, 107, 115, 117 Squid, 24 Stag, 133 Steinach, 140 Stephanosphsera, 5 Stevens, 51 Strobell, 151 Strongylocentrotus, 59, 60, 62 Sturtevant, 72, 98, 117, 118 Stylonichia, 2 Suffolks, 136-138 Synapsis, 93 Tadpoles, 147 Tanager, scarlet, 27 Thomson, 232 Thymus, 146-147 Thyroid, 146-147 Toad, 229 Tower, 117 Toyama, 165 Treat, 232 Triton, 145 Trow, 99 Tschermak, 74 Vermilion eye, 119 Vestigial wing, 96-97 Vigor, 120 Vincent, 146 de Vries, 74, 125 Wallace, 102, 113-114, 120, 125, 127 Wasp, 220 Weismann, 16, 17, 40, 194, 195 Wheeler, 117 White eye, 62-65, 81, 82, 88-92, 118, 119, 221-223 Whitney, 185, 187, 197, 205 Wilder, 237 Wilson, 51 Winiwarter, 245-248 Wood, 136 Woodruff, 8, 198 X-chromosome, 51, 82, 84, 242 Y-chromosome, 51, 84 Yellow body color, 67, 88-92, 119 COLUMBIA UNIVERSITY PRESS Columbia University in the City of New York The Press was incorporated June 8, 1893, to promote the publication of the results of original research. 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