DETERMINATION OF SEX, SECONDARY SEXUAL CHARACTERS, AND SEXUAL INSTINCTS
I. The Cytological Basis of Sex Determination
1. It is a general fact that both sexes appear in approximately equal numbers, provided a sufficiently large number of cases are examined. This fact has furnished the clue for the discovery of the mechanism which determines the relative number of the two sexes. The honour of having pointed the way to the solution of the problem belongs to McClung.[182] It has been known that certain insects, e. g., Hemiptera and Orthoptera, possess two kinds of spermatozoa but only one kind of eggs. The two kinds of spermatozoa differ in regard to a single chromosome, which is either lacking or different in one-half of the spermatozoa.
The first one to recognize the existence of two kinds of spermatozoa was Henking, who stated that in Pyrrhocoris (a Hemipteran) one-half of the spermatozoa of each male possessed a nucleolus, while in the other half it was lacking. Montgomery afterward showed that Henking’s nucleolus was an accessory chromosome. McClung was the first to recognize the importance of this fact for the problem of sex determination. He observed an accessory chromosome in one-half of the spermatozoa of two forms of Orthoptera, Brachystola and Hippiscus, and reached the following conclusion:
A most significant fact, and one upon which almost all investigators are united in opinion, is that the element is apportioned to but one-half of the spermatozoa. Assuming it to be true that the chromatin is the important part of the cell in the matter of heredity, then it follows that we have two kinds of spermatozoa that differ from each other in a vital matter. We expect, therefore, to find in the offspring two sorts of individuals in approximately equal numbers, under normal conditions, that exhibit marked differences in structure. A careful consideration will suggest that nothing but sexual characters thus divides the members of a species into two well-defined groups, and we are logically forced to the conclusion that the peculiar chromosome has some bearing upon the arrangement.
N. M. Stevens and E. B. Wilson[183] have not only proved the correctness of this idea for a number of animals but have laid the foundation of our present knowledge of the subject. Wilson showed that in those cases where there are two types of spermatozoa, one with and one without an accessory or as it is now called an X chromosome, all the cells of the female have one chromosome more than the cells of the male. From this he concludes correctly that in such species a female is produced when the egg is fertilized by a spermatozoön containing an X chromosome, while a male is produced when a spermatozoön without an X chromosome enters the egg.
Such a form is Protenor, one of the Hemiptera. Wilson made sure that all the eggs are alike in the number of chromosomes, each egg containing an X chromosome in addition to the six chromosomes characteristic of the species Protenor. There are two types of spermatozoa in equal numbers in this species, each with six chromosomes, but one with, the other without, an X chromosome. The two possible chromosome combinations between egg and spermatozoa are therefore as follows (see the diagrammatic Fig. [39]):
| Egg | Spermatozoön | Result |
| (1) 6 + X | + 6 | = 12 + X = Male |
| (2) 6 + X | + 6 + X | = 12 + 2X = Female |
The egg which receives a spermatozoön without an X chromosome has after fertilization 12+X chromosomes and develops into a male; while the egg into which a spermatozoön with an X chromosome enters gives rise to a female. Since all the body cells arise from the fertilized egg by nuclear division and the chromosomes remain constant in number in all cells, the consequence is that all the cells of a female Protenor have two X chromosomes; while all the cells of a male Protenor have only one X chromosome.
Fig. 39
The chromosome situation in Protenor is a somewhat extreme case, inasmuch as one X chromosome is entirely lacking in the male. In other forms of Hemiptera, e. g., Lygæus, there are also two types of spermatozoa appearing in equal numbers differing in regard to the X chromosome, but here it is only a difference in size; one-half of the spermatozoa having a large X chromosome, the other half instead a smaller chromosome. Calling this latter the Y chromosome, the sex determination in this form is as follows: leaving aside the chromosomes which are equal in both egg and spermatozoön we may say that there is one type of egg containing one large X chromosome; there are two types of spermatozoa in equal numbers, one possessing a large X chromosome, the other possessing a small Y chromosome. Wilson showed by a study of the chromosomes in males and females that when one of the spermatozoa containing a large X chromosome enters the egg, the egg will develop into a female; while when one of the spermatozoa containing a small Y chromosome enters it will give rise to a male. Leaving aside the common chromosomes of both sexes, a fertilized egg containing XX gives rise to a female, while one containing XY gives rise to a male. There is in this case as in that of Protenor a preponderance of chromosome material in the female, but this quantitative difference is not essential for the determination of sex, since in some species the Y chromosome may be as large as the X chromosome.
The main fact is that the female cells have the chromatin composition XX, the male cells the composition XY, where Y is apparently qualitatively different and often, but not necessarily, smaller than X, or entirely lacking.
It may be mentioned in passing that indirect evidence exists indicating that in man there are also two kinds of spermatozoa and one kind of egg, and that sex depends on whether a male determining or a female determining spermatozoön enters the egg.
2. This mode of sex determination holds only for those animals in which there is one type of egg and two types of spermatozoa. Experimental evidence furnished first by Doncaster in 1908 on a moth, Abraxas, indicated that a number of other forms exists in which matters are reversed, inasmuch as there are two types of eggs and one type of spermatozoa. This condition of affairs exists not only in the moth Abraxas, but also in the fowl as shown by Pearl. In these forms it is assumed that all the spermatozoa have one sex chromosome X, while there are two types of eggs, one possessing the sex chromosome X, the other possessing Y. When a spermatozoön enters an egg with an X chromosome, the egg will give rise to a male, while if it enters a Y egg, a female will arise. The evidence pointing toward this result is chiefly contained in experiments on sex-limited or more correctly sex-linked heredity; i. e., a form of heredity which follows the sex in a peculiar way. Thus colour-blindness is a case of sex-linked inheritance, since this abnormality appears overwhelmingly in the male offspring of a colour-blind person. Doncaster crossed two varieties of Abraxas differing in one character which was sex-linked, and the results of his crossings indicated that in this form there are two types of eggs and one type of spermatozoa.[184]
These observations on sex-linked heredity confirm the idea that the sex chromosomes determine the sex. The most extensive and conclusive experiments along this line are those by Morgan on the fruit fly Drosophila. In this form there are two kinds of spermatozoa and one kind of eggs; the egg has one X chromosome, while one-half of the spermatozoa has an X the other a Y chromosome; the entrance of the latter into an egg gives rise to a male, of the former to a female.
While the eyes of the wild fruit fly Drosophila ampelophila are red, Morgan[185] noticed in one of his cultures a male that had white eyes. This white-eyed male was mated to a red-eyed female. The offspring, the F1 generation, were all red eyed, males as well as females. These were inbred and now gave in the F2 generation the following three types of offspring:
| (1) 50 per cent. females, all with red eyes. | ||
| (2) 50 per cent. males |  | 25 per cent. with red eyes. |
| 25 per cent. with white eyes. | ||
The character white eye was therefore transmitted only to half the grandsons; it was a sex-linked character. It is known from a study of the pedigrees of colour-blind individuals that if the corresponding experiment had been carried out with them, instead of with white-eyed flies, the same proportions of normal and colour-blind would have been found: namely, normal colour vision in the F1 generation, in both males and females, and half of the males of the F2 generation colour-blind, the other half and all the females with normal vision. Of course, in man, intermarriage between two different F1 strains would have been required in place of the inbreeding of the F1 generation, which took place in Morgan’s experiments. Morgan interprets his experiments as follows. The normal red-eyed Drosophila has one kind of eggs, each possessing one X chromosome. This X chromosome has also the factor for the development of red-eye pigment. The white-eyed male has two kinds of spermatozoa, one with an X chromosome, the other with a Y chromosome, both lacking the factor for red-eye pigment. If we designate the X chromosome with the factor for red-eye pigment by X and the X and Y chromosomes lacking the factor for redness with X and Y the following combinations must result if we cross a normal red-eyed female with a white-eyed male:
| Eggs | Sperm | Result |
| X | X | XX red-eyed female |
| X | Y | XY red-eyed male |
It is obvious that all the offspring of the first generation (the F1 generation) must be red eyed, since all the eggs have one X chromosome with the factor for red. According to the results obtained from cytological studies which will be explained in the next chapter, the females with the chromatin constitution XX will form two types of eggs in equal numbers: namely, eggs with an X and eggs with an X, i. e., all eggs have one X chromosome, but in fifty per cent. of the eggs the X has the factor for red, in fifty per cent. this factor is lacking (X). The males having the chromosome constitution XY form two types of spermatozoa, one with an X possessing the factor for red pigment and one, the Y chromosomes, lacking this factor. If inbred the next F2 generation will give rise to the following four types of offspring: (1) XX, (2) XX, (3) XY, (4) XY, all four types in equal numbers.
(1) and (2) give females, both red eyed, since both contain a red-factored X chromosome. (3) and (4) give males, (3) giving rise to red-eyed males, since it contains a red-factored X chromosome, (4) producing males with white eyes since this X chromosome is lacking the factor for red eyes. Since all four combinations must appear in equal numbers (provided the experimental material is ample enough, which was the case in these experiments), in the F1 generation both males and females should have red eyes and in the F2 generation all the females should have red eyes and half of the males should have red, half white eyes. These results were obtained.
The experiments were carried further. No white-eyed females had appeared thus far. On the same assumptions of the relation of the X, X, and Y chromosomes to the heredity of sex as well as to eye colour it was possible to predict under what conditions and in which proportions white-eyed females should arise. Thus if a red-eyed female of the F1 generation (a cross between white-eyed male and normal female) be mated with a white-eyed male the result should be an equal number of white-eyed males and white-eyed females if the chromosome theory of sex determination were correct. The reasoning would be as follows:
The red-eyed female, having the chromosome constitution XX should form two kinds of eggs in equal numbers with the constitution X and X; the white-eyed male having the chromosome constitution XY should form two kinds of spermatozoa X and Y. The following four types of individuals must then be produced in equal numbers:
(1) XX, (2) XX, (3) XY, and (4) XY.
In this case (2) must give rise to white-eyed females and (4) to white-eyed males, while (1) must give rise to red-eyed females and (3) to red-eyed males. Hence white-eyed males and females and red-eyed males and females are to be expected in this case in equal numbers, and this was actually observed.
The numerical agreement in this and the other experiments between the expected and observed result cannot well be an accident. The fact that the inheritance of sex-linked characters in man follows the same laws as in Drosophila is a strong argument in favour of the assumption that in man, also, sex is determined by two kinds of spermatozoa.
Morgan and his students discovered no less than thirty-six sex-linked characters in Drosophila, and each behaved in a similar way to the red and white eye colour in regard to sex-linked inheritance, so that the chromosome theory of sex determination rests on a safe basis. That sex is merely determined by the number of X chromosomes, not by the Y chromosome, is proved by the facts that the Y chromosome may be completely absent as in Protenor and that Bridges[186] has found a type of female Drosophila with a chromosome formula XXY whose sex was not affected by the supernumerary Y.
3. On the basis of all these experiments and theories it is comparatively easy to explain a number of phenomena concerning sex ratios which before had been very puzzling. In bees it had been shown many years ago by Dzierzon that the males develop from unfertilized eggs while the females, queens and workers, develop from fertilized eggs. This is intelligible on the assumption that the unfertilized egg contains only one X chromosome while the spermatozoön carries into the egg the second X chromosome. But if the male bee produces two types of spermatozoa we should expect that only one-half of the fertilized eggs should be females, the other half males. But it happens that of the two types of spermatozoa only one is formed since in one of the cell divisions which lead to the formation of spermatozoa one viable spermatozoön only is formed while the other one perishes. It is, therefore, quite possible that it is the female-producing spermatozoön which survives while the male-producing spermatozoön dies.
It is occasionally observed that an insect shows one sex on one side of its body and the opposite sex on the other side. Boveri suggested that this phenomenon of gynandromorphism is due to the fact that the spermatozoön for some unknown reason does not fuse with the egg nucleus until after the egg has undergone its first cell division. In this case it fuses with the nucleus of one of the two cells into which the egg divides (or in some cases even one of the later cells?). As a consequence the one-half of the embryo which arises from the cell which was not fertilized would have only one X chromosome and in a case like the bee would develop parthenogenetically, while the other half of the body, developing from the cell into which a spermatozoön has penetrated, would be fertilized. The latter half of the body would be female, the former male. In his last paper before his untimely death, Boveri has given proof for the correctness of this interpretation as far as gynandromorphism in the bee is concerned.[187]
It seems to be generally true that where sexual reproduction leads only to the formation of females the case finds its explanation in the fact that the male-producing spermatozoa perish and only the female-producing spermatozoa survive. Such an observation was made by Morgan on a certain species of phylloxerans.
The slight preponderance in the number of one sex which is occasionally found—an excess of six per cent. males over females in the human race—may well find its explanation on the assumption of a slightly greater mortality of the female-determining spermatozoa.
In certain forms parthenogenetic and sexual reproduction may alternate in a cycle, e. g., in plant lice, Daphnia, and rotifers. In plant lice it has been observed for a long time that when the plant is normal and the weather warm the aphides remain wingless, reproduce parthenogenetically, and only females exist, and this may last for years and for more than fifty generations; but that when the plant is allowed to dry out both sexes appear.
Here we are dealing with a limited determination of sex inasmuch as the experimenter has it in his power to prevent or allow the production of males. The facts do not in all probability contradict the statements made concerning the rôle of the X chromosomes in the determination of sex. We have seen that where sex is determined by two types of spermatozoa one type of eggs is produced which possesses only one X chromosome. Such eggs might produce males if not fertilized (as they do in bees), but they cannot produce females because for that purpose they must have two X chromosomes. It has been shown for certain cases, and it may be true generally, that if eggs of this type give rise to parthenogenetic females they may do so because they have for some reason two X chromosomes. Usually such an egg loses one of the X chromosomes in a process of nuclear division (the so-called reduction division) which usually precedes fertilization. If this reduction division is omitted the egg has two X chromosomes and if such an egg develops parthenogenetically it gives rise to a female. These cases do not, therefore, contradict the connection between X chromosomes and sex determination established by cytological observations and breeding experiments, on the contrary, they confirm it. The question remains: How can external conditions bring it about that the reduction division is omitted? To this question no definite answer can be given at present.
We may in passing mention the well-known observation that twins which originate from the same egg always have the same sex; while twins arising from different eggs show the usual variation as to sex. Twins coming from one egg have the same chorion and can thereby be diagnosed as such. They can be produced as we have stated in Chapter V by a separation of the first two cleavage cells of the egg, each one giving rise to a full embryo. It harmonizes with all that has been said above that the sex of two such individuals must be the same since they have the same number of X chromosomes, the latter being determined in the human race by the nature of the spermatozoön which enters the egg.
4. While thus far all the facts agree with the dominating influence of certain chromosomes upon sex determination, one group of facts has not yet been explained: namely, hermaphroditism. By hermaphroditism is meant the existence of complete and separate sets of female and male gonads in the same individual. This condition exists regularly not only in definite groups of animals, e. g., certain snails, leeches, tape-worms, but also, as everybody knows, in flowering plants. While in some forms both kinds of sex cells, male and female, are formed and mature simultaneously, as, e. g., in the Ascidian Ciona (see Chapter IV), in others they are formed successively, very often the spermatozoa appearing first (protandric hermaphroditism). In the long tapeworm Tænia each ring has testes and ovaries, but the young rings are only male while in the older rings the testes disappear and the ovaries are formed. The same ring is in succession male and female. How can we reconcile the facts of hermaphroditism with the chromosome theory of sex determination? Rhabdonema nigrovenosum, a parasite living in the lungs of the frog, is hermaphroditic, but its eggs produce not a hermaphroditic generation but one with the two separate sexes; this generation is not parasitic and lives in the soil. The generation produced by these separate males and females gives rise again to a hermaphrodite which migrates into the lungs of the frogs. According to Boveri and Schleip[188] the cells of the hermaphrodite have twelve chromosomes. It produces two types of spermatozoa with six and five chromosomes respectively (one-half of the cells losing one chromosome which is left at the line of cleavage between the two cells); and one type with six chromosomes. In this way separate males and females are produced by the hermaphrodite, females with twelve and males with eleven chromosomes.
The males produce again two kinds of spermatozoa, male and female producing, but the male-producing spermatozoa become functionless. This fusion of the other spermatozoön containing six chromosomes with an egg having six chromosomes leads again to the formation of the hermaphrodite with twelve chromosomes. It is obvious that in this case the cause for the hermaphroditism is not disclosed. If chromosomes have anything to do with hermaphroditism there must be an undiscovered element in the chromosomes which may explain why the female as well as the hermaphrodite have the same chromosome constitution; or we are forced to look for another determinant outside the X chromosomes or the chromosomes altogether. This seems to be the only cytological work on the problem of hermaphroditism. Experimental work has been begun by Correns[189] and by Shull on the determination of hermaphroditism in plants but lack of space forbids us to give details.
II. The Physiological Basis of Sex Determination
5. As stated at the beginning of this chapter, the chromosome theory of sex determination explained only one feature of the problem, namely, the relative numbers in which both sexes or only one sex, as the case may be, are produced; and in this respect the evidence is so complete that we must accept it. But with all this, the problem of sex determination is not exhausted, since a physiological solution of the problem of sex determination demands an account of how the sex chromosomes can induce the formation not only of ovaries and testes but also of the other sex characters. For the solution of this problem biology will have to depend largely on experiments in which it is possible to influence the formation of sex characters and of the sex glands themselves.
The most striking observations in this direction were made by Baltzer on a marine worm, Bonellia. In this animal the two sexes are very different, the male being a tiny parasite, a few millimetres in length, which spends its life in the uterus of the female, whose size is about five centimetres. A female carries as a rule several and often a large number of the male parasites in its uterus, which indicates that the males prevail numerically. The fertilized eggs of the animals are laid in the sea water where the larvæ hatch. At the time of hatching all larvæ are alike. The differentiation of the larvæ into the dwarf males and the giant females can be determined at will. The larvæ have a tendency to attach themselves to the proboscis of the female as soon as they hatch. If given a chance to do so and if they stick to the proboscis for more than three days they will develop into males, which soon afterwards creep into the female where they continue their parasitic existence. If, however, no adult female Bonellia is put into the aquarium in which the larvæ hatch, about ninety per cent. of the larvæ will, after a period of rest, develop into females; the rest develop into males. Those which develop into females will often show a primary maleness which may manifest itself in the production of sperm or of other secondary male sexual characters. This tendency is stronger the longer the period of rest lasts. If the larvæ are allowed to settle on the proboscis of the adult female but are removed too early hermaphrodites are produced having male and female characters mixed.
Baltzer has suggested on the basis of some observations that the larvæ while on the proboscis of the female absorb some substance secreted by the proboscis, and this substance accelerates the further development into a male and suppresses the female tendency. If this substance from the proboscis does not reach the larvæ the tendency to become males is gradually suppressed in the majority and only a few develop into pure males or protandric hermaphrodites, while the female characters are given a chance to develop. Baltzer assumes, therefore,—as it seems to us correctly—that in all larvæ the tendency for both sexual characters is present, that they are, in other words, hermaphrodites, but the chance for the suppression of one and the development of the other group of characters can be influenced by certain chemical substances which the larva may take up.[190]
Giard has studied the effects of a curious form of castration brought about by parasites, which is followed by a change in the sexual character of the castrated animal. The phenomenon is very striking in certain forms of crabs when they are attacked by a parasitic crustacean, Sacculina. The two sexes differ in the crab Carcinus mænas by the form of the abdomen, but when a male is attacked by the parasite its abdomen assumes the female shape. Smith observed in another crab that in such cases even the abdominal appendages of the male may be transformed into those of a female. The transformation is so complete that the older observers had reached the conclusion that the parasite attacked only the females, since they overlooked the fact that the castration by the parasite transformed the secondary sexual characters of the male into those of a female.
Giard observed that in a diœcious plant, Lychnis dioica, a parasitic fungus brings about the transformation of the host into a hermaphrodite.
G. Smith has discovered a fact which shows that chemical changes must underlie these morphological transformations of primary or secondary sexual characters. He noticed that in male crabs the presence of the parasite Sacculina changes the contents of the fatty constituents in the blood, making them equal to that of the female. Vaney and Meignon had previously shown that during the chrysalid stage the female silkworms have always more glycogen and less fat than the males. The castration by parasites is paralleled by what Caullery calls the castration by senility.[191] In certain birds and also in mammals at the time when the sexual glands cease to function certain secondary sexual characters of the other sex make their appearance. The most common case is that certain secondary male characters appear in the old female (exceptionally also in the young female with abnormal ovaries) (arrhenoidy). Thus old female pheasants assume the plumage of the male, and in the human female after the menopause and especially among sterile women a beard may begin to grow. The opposite phenomenon, the old male assuming female characters, is not so common. Very interesting observations on changes in the plumage of castrated fowl have recently been made by Goodale.[192]
It had long been observed by cattle breeders that in the case of twins of different sex the female—the so-called free-martin—is usually sterile. F. Lillie[193] has recently discovered the cause of this interesting phenomenon. Such twins originate from two different eggs since the mother has two corpora lutea, one in each ovary. In normal single pregnancies in cattle there is never more than one corpus luteum present. The two eggs begin to develop separately in each horn of the uterus.
The rapidly elongating ova meet and fuse in the small body of the uterus at some time between the 10 mm. and the 20 mm. stage. The blood-vessels from each side then anastomose in the connecting part of the chorion; a particularly wide arterial anastomosis develops, so that either fetus can be injected from the other. The arterial circulation of each also overlaps the venous territory of the other, so that a constant interchange of blood takes place. If both are males or both are females no harm results from this; but if one is male and the other female, the reproductive system of the female is largely suppressed, and certain male organs even develop in the female. This is unquestionably to be interpreted as a case of hormone action.
The reproductive system of these sterile females is for the most part of the female type, though greatly reduced. The gonad is the part most affected; so much so that most authors have interpreted it as testis.
It should be added, however, that this result cannot at present be generalized, since in the hermaphrodites the specific hormones of both sexes must circulate without suppressing each other’s efficiency.
All these facts indicate that certain substances secreted by the ovaries or testes may inhibit the development of certain sexual characters of the opposite sex. When these inhibitions are partly or entirely removed the secondary sexual characters of the opposite sex may appear. This fact may also be interpreted as an indication of a latent hermaphroditism and if this be correct the real and latent hermaphrodites differ only by the degree of inhibition for one sex, this inhibition being lacking or less complete in the real than in the latent hermaphrodite.
In the light of this conclusion the observations on the regeneration of both ovaries and testicles which Janda observed in a hermaphroditic worm, Criodrilus lacuum,[194] is no longer so mysterious. This worm normally possesses in the segments near the head a pair of ovaries and several pairs of testes. Janda found that if the anterior parts containing the gonads of these worms are cut off a complete regeneration takes place, including both types of gonads, ovaries as well as testes. As a rule, more than one pair of ovaries appear in the regenerated piece. This important experiment shows that in a hermaphrodite both types of sex organs can be produced from body cells or from latent buds resembling body cells. This phenomenon would be intelligible on the assumption that in the body of a hermaphrodite substances circulate which favour the development of both types of sex organs, while in a diœcian animal probably only one type of sex organ would be developed; the formation of the other being inhibited.
Richard Goldschmidt has discovered in his breeding experiments on the gipsy-moth (Lymantria dispar) a phenomenon which will probably throw much light on the physiology of sex determination. He found that certain crosses between the Japanese and the European gipsy-moth do not give pure sexes, males or females, but mixtures of the sexual characters of both sexes, and this mixture is a very definite one for definite crosses. These differences are such that it is possible to grade the hybrids according to their manifestations of maleness or femaleness, both in morphological characters and instincts. Goldschmidt calls this peculiar phenomenon intersexualism, and its essential feature is that the various degrees of intersexualism can be produced at will by the right combination of races.
Female intersexualism begins with animals which show feathered antennæ of medium size (feathered antennæ are a male character), but which are otherwise entirely female in appearance except that they produce a smaller number of eggs which are fertilized normally. In the next stage patches of the brown male pigment appear on the white female wings in steadily increasing quantity. The instincts are still female, the males are attracted and copulate. But the characteristic egg sponge laid by the animal contains nothing but anal hairs in spite of the fact that the abdomen is filled with ripe eggs. In the next stage whole sections of the wings show male colouration, with cuneiform female sectors between, the abdomen becomes smaller, contains fewer ripe eggs, the instincts are only slightly female, the males are attracted very little, and reproduction is impossible. In the next stage the male pigment covers practically the whole wing, the abdomen is almost male, but still contains ovaries with a few ripe eggs, the instincts are intermediate between male and female. Then follow very male-like animals which still show in different organs their female origin and have rudimentary ovaries. . . . The end of the series is formed by males, which show in some minor characters, such as the shape of wings, still some traces of their female origin.
The series of the male intersexes starts with males showing a few white female spots on their wings. These become larger and larger, the amount of brown pigment correspondingly decreasing. . . . Hand in hand with this the abdomen increases in size, reaching in the most extreme cases two-thirds of the female size (without containing eggs). The same is true for the instincts which become more and more female.
(And also for the copulatory organs which also become more and more female.)
As stated above, the main fact that every desired degree of intersexualism can be produced at will by properly combining the races for breeding, and the intersexual potencies of the different races has been worked out by Goldschmidt.[195]
6. The relation between chemical substances circulating in the body—either derivatives of food taken up from without or of chemical compounds formed naturally inside the body—and the production of sexual characters is best shown in the polymorphism found among the social ants, bees, and wasps. Here we have, as a rule, in addition to the two sexes a third one, the workers, which are in reality rudimentary and for that reason sterile females. They differ more or less markedly from both the typical male and female in their external form, and, as a rule cannot copulate owing to their deficient structure. This third sex, the sterile neuters, can be transformed at desire into sexual females in certain species, as P. Marchal has demonstrated. He worked with a form of social wasps in which the workers are sterile and smaller than the real females. In such a society of wasps all the males and workers die in the fall and only the fertilized females survive, each one founding a new nest in the following spring. From the first eggs laid, workers arise, small in stature and sterile; these workers are nourished by their mother. Then these workers take care of the feeding of all those larvæ which arise from the eggs which their mother continues to lay. Throughout the spring only workers arise from the eggs. The males appear in the summer, the real females towards the end of the season when the sexes copulate.
Marchal isolated a number of the sterile workers, providing them with food but giving them no larvæ to raise. He found that the workers which thus far had been sterile became fertile, producing, however, only males. This latter fact is easily understood from what has been said regarding the bees, namely, that the female produces only one type of eggs, hence the unfertilized egg can give rise only to males. The astonishing or important point is that the ovaries of the workers begin to develop as soon as they no longer have a chance to nourish the larvæ, provided the food which would have been given to the larvæ is now at their disposal. In other words, the development of their ovaries is the outcome of eating the food which under normal conditions they would have given to the larvæ. The food must, therefore, contain a substance which induces the development of eggs. The natural sterility of the neuters or workers is, therefore, to use P. Marchal’s expression, a case of “food castration,” (“castration nutriciale”).[196] The workers originate from fertilized eggs and are therefore females, but for the full development of the ovaries and the other sexual characters something else besides the XX chromosomes is needed and this is supplied in this case by the quantity or quality of the food. May we not conclude that the same thing may happen generally, except that these substances are formed by the body under the normal conditions of nutrition through the influence of constituents of the second X chromosome?
It is known that the future queens among the bees receive also a special type of food which the workers do not receive. Again the idea of “food castration” of the latter is suggested.
In rotifers Whitney[197] has shown that the cycle in the production of males and females can be regulated by the food. In some species a scanty supply of green flagellates produced purely female offspring, while a copious diet of the same green flagellates produced a predominance of male grandchildren, sometimes as high as ninety-five per cent. This was confirmed by Shull and Ladoff.[198]
7. The effects of the removal of the ovaries or testes upon the development of secondary sexual characters differ for different species. In insects the secondary sexual characters are not altered by an operative removal of the sexual glands as in the caterpillar, e. g., Ocneria dispar, according to Oudemans. This result has been invariably confirmed by all subsequent workers, especially by Meisenheimer. Crampton grafted the heads of pupæ of butterflies upon the bodies of other specimens of the opposite sex, but the sexual characters of the head remained unaltered.
In vertebrates, however, there exists a distinct influence of a secretion from the sexual glands upon the development of certain of the secondary sexual characters, which do not develop until sexual maturity. In a way the observations on arrhenoidy and thelyidy referred to above are indications of this influence.
Bouin and Ancel had already suggested that the sexual glands of mammals have two independent constituents, the sexual cells and the interstitial tissue; and that the latter tissue is responsible for the development of the secondary sexual character. This has been proved definitely by Steinach,[199] who showed that when young rats are castrated certain secondary sexual characters are not fully developed. The seminal vesicles and the prostate remain rudimentary and the penis develops incompletely. Such animals when adult recognize the female and seem to follow it, but do not persist in their attention and neither erection nor cohabitation occurs. When, however, the testes are retransplanted into the muscles of the castrated young animal (so that they are no longer connected with their nerves) seminal vesicles, prostate, and penis develop normally, and these animals show normal sexual ardour and cohabitate with a female although the female cannot become pregnant since the males cannot ejaculate any sperm. When the retransplanted testes were examined it was found that all the sperm cells had perished, only the interstitial tissue of the testes remaining. It was, therefore, proved that the development of the seminal vesicles, the prostate, the penis, and the normal sexual instincts and activities depends upon the internal secretions from this interstitial tissue and not upon the sex cells proper. This agrees with the conclusions at which Bouin and Ancel had arrived by ligaturing the vasa deferentia of male animals.
Steinach in another series of experiments castrated young male rats and transplanted into them the ovaries of young females. These ovaries did not disintegrate, the eggs remaining, and corpora lutea were formed. In such feminized individuals the seminal vesicles, prostate, and penis did not reach their normal development, and it was thereby proved that the internal secretions from the ovary do not promote the growth of the secondary sexual male characters. On the contrary, Steinach was able to show that the growth of the penis was directly inhibited by the ovary, since in the feminized males this organ remained smaller than in the merely castrated animals. On the other hand the infantile uterus and tube when transplanted into the young male with the ovaries grow in a normal way, and Steinach thinks that pregnancy in such feminized males is possible if sperm be injected into the uterus. In some regards the feminized males showed the morphological habitus of females. Soon after the transplantation of ovaries into a castrated male the nipples of its mammary glands begin to grow to the large size which they have in the female and by which the two sexes can easily be discriminated. In addition the stronger longitudinal growth of the body in the male does not occur in the feminized specimens, the body growth becomes that of a female; and likewise the fat and hair of the feminized male resemble that of a real female.
While the castrated males show an interest in the females, the feminized males are absolutely indifferent to females and behave like them when put together with normal males; and, what is more interesting, they are treated by normal males like normal females. The sexual instincts have, therefore, also been reversed in the feminized males by the substitution of ovaries for testes.
The inhibition of the growth of the penis by the ovary is of importance; it supports the idea already expressed that in hermaphrodites this inhibition of the growth of the secondary organs of the other sex is only feeble or does not exist at all.
We may finally ask whether there is any connection between the cytological basis of sex determination by special sex chromosomes and the physiological basis of sex determination by specific substances or internal secretions. It is possible that the sex chromosomes determine or favour, in a way as yet unknown, the formation of the specific internal secretion discussed in the second part of this chapter. In this way all the facts of sex determination might be harmonized, and it may become clear that when it is possible to modify secretions by outside conditions or to feed the body with certain as yet unknown specific substances the influence of the sex chromosomes upon the determination of sex may be overcome.