128 HEREDITY [ch. 



from a moth and two kinds of birds shows that in 

 them the female produces two kinds of germ-cells. 



The conclusion has been drawn by some students 

 of the subject that in the Moth, Canary and Fowl the 

 egg determines the sex of the offspring, while in Man 

 and the fly Drosophila the spermatozoon determines 

 the sex. There is some evidence, however, that both 

 modes of inheritance may exist in Fowls, which would 

 contradict this supposition, and for this and other 

 reasons it is perhaps more likely that in animals 

 generally, and probably in plants also, there are two 

 kinds of egg-cells (germ-cells produced by the female) 

 and two kinds of spermatozoa (germ-cells of the male) 

 and that an egg-cell destined to produce a female is 

 fertilised by a spermatozoon of one kind, a male- 

 producing egg by a spermatozoon of the other kind. 

 The facts in any case indicate that the determiners 

 for maleness and femaleness are comparable with 

 Mendelian characters, which segregate from each 

 other in the formation of the germ-cells as do the 

 'factors' for Mendelian allelomorphs. For a more 

 detailed presentation of this hypothesis and the 

 evidence on which it rests, the reader must be referred 

 to the works cited in the bibliography [especially 2, 

 9a, 226, 43a]. 



It will be noticed that in the description of the 

 nature of sex-determination here barely outlined, the 

 sex of an individual is regarded as being no less 



