CHAPTER VIII

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. 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.

McClung, C. E., “The Accessory Chromosome--Sex Determinant?” Biol. Bull., 1902, iii., 43.

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 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.

Wilson, E. B., “Studies on Chromosomes,” Jour. Exper. Zoöl., 1905, ii., 371, 507; 1906, iii., 1; 1909, vi., 69, 147; 1910, ix., 53; 1912, xiii., 345. “Croonian Lecture,” 1914, Proc. Roy. Soc., B. lxxxviii., 333.

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.

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.

Doncaster, L., The Determination of Sex. Cambridge, 1914.

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 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:

Morgan, T. H., Heredity and Sex. New York, 1913.

(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 =X=X red-eyed female

=X= Y =X=Y 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 =X=X 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 =X=Y 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) =X==X=, (2) =X=X, (3) =X=Y, (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 F₂ 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 F₂ 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 =X=X 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) =X=X, (2) XX, (3) =X=Y, 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 has found a type of female Drosophila with a chromosome formula XXY whose sex was not affected by the supernumerary Y.

Bridges, C. B., Genetics, 1916, i., 1.

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.

Boveri, Th., Arch. f. Entwcklngsmech., 1915, xlii., 264.

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