202 GENETICS 



Similarly, in the clialcid fly, Ageniapsis, sl chain of 

 embryos is formed from a single egg, and these, ac- 

 cording to Marschal, are all of the same sex. 



Newman and Patterson also have shown that in 

 the armadillo, Tatusia, there are customarily produced 

 four young within a single chorion, all of which are 

 of the same sex. 



These facts point toward the conclusion that the 

 determination of sex takes place at the time of fer- 

 tilization. 



5. Selective Fertilization 



Within the last ten years considerable evidence 

 has been collected in support of the supposition that 

 sex is a Mendelian character. Mendel himself, 

 without elaborating this idea into a definite hypothe- 

 sis, suggested the probability that sex is a heritable 

 character behaving in the same way as other herit- 

 able characters. 



In 1903 Castle published a paper ^ in which a 

 tentative explanation, since abandoned, of the phe- 

 nomenon of sex determination was advanced, based 

 upon three assumptions : first, that all germ-cells are 

 heterozygous for sex and, therefore, upon maturation 

 there are formed both male and female eggs as well 

 as male and female sperms ; second, that in fertili- 

 zation the gametes always unite with their opposites 

 so far as sex is concerned ajid never with their like, 

 with the result that each fertilized egg must carry 



1 Castle, W. E., "The Heredity of Sex." Bull. Mus. Comp. Zool., 

 Harvard, Vol. XL, No. 4, 1903. 



