Lnternal 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 + 1FF 
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 usno 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 view 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 Parthenogenetic Eggs 
The question of the number of the polar bodies extruded in 
parthenogenetic eggs has also played a conspicuous réle 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 spermatozoén, 
