202 GENETICS 



Similarly, in the chalcid fly, Ageniapsis, a 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 l 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 and never with their like, 

 with the result that each fertilized egg must carry 



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

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



