Basis of Sex Determination 203 



It may be mentioned in passing that indirect evidence 

 exists indicating that in man there are also two kinds 

 of spermatozoa and one kind of egg, and that sex 

 depends on whether a male determining or a female 

 determining spermatozoon enters the egg. 



2. This mode of sex determination holds only for 

 those animals in which there is one type of egg and two 

 types of spermatozoa. Experimental evidence furnished 

 first by Doncaster in 1908 on a moth, Abraxas, indi- 

 cated that a number of other forms exists in which 

 matters are reversed, inasmuch as there are two types 

 of eggs and one type of spermatozoa. This condition of 

 affairs exists not only in the moth Abraxas, but also 

 in the fowl as shown by Pearl. In these forms it is 

 assumed that all the spermatozoa have one sex chromo- 

 some X, while there are two types of eggs, one possessing 

 the sex chromosome X, the other possessing Y. "When 

 a spermatozoon enters an egg with an X chromosome, 

 the egg will give rise to a male, while if it enters a Y 

 egg, a female will arise. The evidence pointing toward 

 this result is chiefly contained in experiments on sex- 

 limited or more correctly sex-linlvcd heredity; i. e., a 

 form of heredity which follow\s the sex in a peculiar 

 way. Thus colour-blindness is a case of sex-linked 

 inheritance, since this abnormality appears over- 

 whelmingly in the male offspring of a colour-blind 

 person. Doncaster crossed tw^o varieties of A braxas dif- 

 fering in one character which w^as sex-linked, and the 



