124 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in the female and sperm-production in the male as secondary conse- 

 quences. It is interesting to speculate whether the characteristics of 

 ovum and female, of spermatozoon and male, may not each be due to 

 fundamental ratabolic and anabolic tendencies, characteristic of male- 

 ness and femaleness, quite apart from the exigences of reproduction, as 

 suggested by Geddes. 



There have been three main opinions as to the true period of sex- 

 determination, the oldest being that this takes place in embryonic life, 

 while more recent investigations have pointed to the time of fertilization 

 or before that. In Phylloxera and Hydatina two kinds of eggs are 

 laid, which differ from one another in size, and the larger always pro- 

 duce females, the smaller males. The two kinds of eggs are laid by 

 different parthenogenetic mothers, and the same distinction into male- 

 producing and female-producing females is of wide, though not invari- 

 able occurrence in Aphids, Rotifers, and Cynipidte. In the last two 

 fertilized eggs always yield females, while parthenogenetic eggs are of 

 two sorts, male-producing and female-producing. In Rotifers, an egg 

 which would have yielded a male if unfertilized, produces a female if 

 fertilized. 



In regard to Dinophilus, it used to be believed that the two sizes of 

 eggs both required fertilization, but the work of Shearer has shown that 

 the larger female-producing eggs conjugate with a sperm-nucleus, while 

 the smaller male-producing eggs do not. In the hive-bee and some 

 other Hymenoptera unfertilized eggs develop into males. In some 

 moths and birds the female transmits certain characters to her male 

 offspring only : therefore there must be two kinds of eggs. In some 

 Mammals (cat and man) the male transmits certain characters to the 

 female offspring exclusively : therefore, according to Doncaster, there 

 must be two kinds of spermatozoa. 



The facts of " sex-limited inheritance " (notably in currant-moth, 

 Drosophila, canary, cat, and man) seem to show that sex-determining 

 factors are borne by the ova and spermatozoa, and that sex cannot 

 be altered after fertilization. It is curious that some of the cases 

 point to the egg being all-important as regards sex-determination, while 

 others point to the spermatozoon. It is possible that sex is determined 

 by an interaction of factors from the two parents. 



The evidence is then adduced in support of the hypothesis that the 

 presence of an unpaired or unequally paired chromosome is connected 

 with sex-limited inheritance. It is not maintained, however, that the 

 sex is determined simply and immediately by the presence or absence of 

 a particular chromosome, for it may be that chromosomes are rather 

 links in a chain of events, of which the determination of sex is only 

 one. 



If there are two kinds of spermatozoa in some types and two kinds of 

 eggs in others, and if these two kinds are produced in equal numbers, as 

 there is good reason to expect, the numbers of the sexes should be 

 approximately equal. But the ratio at birth of males to females varies 

 considerably. It may be that the proportions of the two kinds of eggs 

 can be altered by environmental influences ; it may be that the state of 

 the germ-cells at fertilization is important ; it may be that effects of 



