Internal Factors of Sex Determination 415 



of the latter as of the other two combined, as shown in the 

 formula : 



Spermatozoa M + F 



Eggs M + F 



iMM + 2MF + iFF 



Since we get no such results in animals, the Mendelian scheme 

 must evidently be altered if it is to be made use of as an hypothe- 

 sis of sex. Therefore Castle assumes that male spermatozoa 

 can fertilize only female eggs, and that female spermatozoa can 

 fertilize male eggs. Hence every individual is, as it were, a sexual 

 hybrid, since it contains both male and female elements. But 

 what determines the sex of the individual? Castle's hypoth- 

 esis gives us no answer. It seems to me, therefore, that despite 

 the assumptions of male and female spermatozoa, and of male 

 and female eggs, and the further assumption of selective fertili- 

 zation, the hypothesis does not advance us toward an explana- 

 tion of sex determination. If it be claimed that the hypothesis 

 deals only with the heredity of sex rather than with its determi- 

 nation (although Castle places no such limitation on his view), 

 it is still not apparent what the hypothesis accomplishes, since 

 it assumes a separation of the sex-elements (which is entirely 

 hypothetical) only in order to bring them together again by means 

 of selective fertilization (likewise an assumption). Since we 

 have some evidence in favor of the >iew that both sex elements 

 are carried by the sperm as well as by the egg, the heredity of 

 sex is accounted for without any additional hypotheses. 



The Reduction Process in Partheno genetic Eggs 



The question of the number of the polar bodies extruded in 

 parthenogenetic eggs has also played a conspicuous role in 

 modern speculation concerning sex determination. Weismann 

 was led by certain theoretical considerations to examine the eggs 

 of parthenogenetic species. He found only one polar body given 

 off in the parthenogenetic eggs of daphnia. This led to the 

 idea that the second polar body may act like the spermatozoon, 



