240 



DISCOVERY 



factors in the Japanese race are geared at a higher 

 rate than those of the European race — are producing 

 more of the characteristic sex-determining substances 

 in a given time. Let us mark this difference by 

 calling the sex-factors of the Japanese race X+ and 

 Q + , those of the European race X^ and Q~. 



The cross Japanese male X European female will, 

 as a minute's calculation will show, give female off- 

 spring with a "strong" male and a "weak" female 

 factor — X+ combined with Q~. Both male and 

 female factors are producing their determining sub- 

 stances; in a normal female Q keeps ahead of X in 

 its production; but here the "strong" X4- is nroduc- 

 ing too fast for the "weak" Q— , and after a lime 

 catches it up. From this time forward, there is an 

 excess of male-determining substances, and the 

 animal must finish its growth as a male. The 

 opposite result, when the production of female- 

 determining substance gradually catches up and sur- 

 passes that of the male-determining, only seems to 

 occur \\hen a "strong" female factor, Q-t- is combined 

 with two "weak" male-producing factors, X~ X~, 

 and this can only come about in the second genera- 

 tion after a cross. 



Similar reversals of sex are known in other animals, 

 in shrimps, in frogs, and probably in hens. Indeed, 

 it is quite possible that some sexually abnormal 

 human beings are the victims of this sex-reversing 

 power, and deserving, not of the opprobrium which 

 they generally receive, but of pity for being in the 

 grip of inexorable hereditary forces. 



There is one curious consequence of intersexuality. 

 If it goes so far as to lead to complete reversal, an 

 animal will be produced with the appearance and 

 functions of one sex, but the chromosome-constitu- 

 tion proper to the other. When such an animal 

 comes to reproduce, this must lead to upsets of the 

 sex-ratio in the next generation. In frogs, for 

 instance, the sex chromosomes are probablv XX in 

 the female, XY in the male. When a female is 

 converted into a functional male, she (or he, as we 

 should now say) will still have the two X's. There- 

 fore, when this animal mates with a normal female, 

 which will also possess two X chromosomes, all the 

 sperms and eggs alike will contain an X, and all the 

 offspring will therefore be XX in constitution, and 

 therefore females. Such an experiment has actuallv 

 been carried out, and nothing but females obtained 

 among the seven hundred odd offspring. 



Space forbids us to enumerate more of these 

 experiments, for, remarkable as some of them are — 

 for instance, the fact that if a frog which has started 

 to lay is stopped, and the rest of the eggs fertilised 

 three or four days later, they will all give rise to 

 males — we have as yet no explanation for them. 



We must conclude with a short consideration of 

 sex-determination in mammals and man. As we 



might expect, here too sex is normallv determined at 

 the moment of fertilisation. This is shown particu- 

 larly well bv the fact that whenever a single fertilised 

 egg divides so as to give rise to two or more off- 

 spring, these are always of the same sex. This 

 occurs as a normal event in the Texas Armadillo, as 

 an exception in so-called "identical" twins in man. 

 Other twins, and the members of the same litter in 

 most animals, are equally likelv to be of opposite- 

 sexes. 



One of the most puzzling things so far discovered 

 about sex is that the ratio of males to females is not 

 always equal, and may vary from species to species, 

 and at different seasons of the year. The fact that 

 one sex possesses two, the other one X-chromosomc 

 should inevitably produce equal numbers of males and 

 of females. Yet in man, for instance, the ratio of 

 male to female births is about 107 to 100, and if we 

 take into account the embryos which die before birth, 

 the ratio of male to female conceptions is about 

 130 to 100. In many breeds of animals, again, the 

 percentage of females is greater when the breeding 

 season is at its height, the percentage of males 

 greater whilst fewer young are being born. Recently 

 it has been asserted by Siegel, on the basis of a good 

 deal of evidence, that in man the percentage of males 

 varies very considerably according to the time during 

 the monthly period at which fertilisation or concep- 

 tion takes place. If this proves to be true, the 

 power of controlling the sex of our children, of 

 having boys or girls at will, will be to a considerable 

 extent within our grasp. But in face of the fact that 

 a great many other so-called specifics for controlling 

 sex have turned out to be worthless, it is as well to 

 be cautious. It has been maintained, for instance, 

 that the right ovary produces eggs giving rise to 

 boys, the left those which give rise to girls ; or that 

 the formation of male-producing and female-produc- 

 ing eggs alternate, first one sort and next time the 

 other being given off. But it has been definitely 

 shown that there is no foundation for these and manv 

 other similar statements. 



There is, however, one understood fact which mav 

 prove to explain many difficulties. As we have 

 already seen, male mammals have but one X chromo- 

 some. Their sperms therefore are of two kinds, one 

 with, the other without an X. \\'here careful 

 examiucition has been made, it is found that these 

 two sorts of sperms can be distinguished by the micro- 

 scope, those without an X being smaller. It is quite 

 possible that these smaller sperms may be more 

 delicate, or, in their long journey to reach the ovum 

 within the female's body, may swim at a different 

 rate from the larger. In any case, anything which 

 affected the two classes of sperms difterentlv wcjuld 

 lead to a difference in the proportion of males and 

 females produced. 



