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THE AMERICAN NATURALIST [Vol. XL V 



ing only one sex factor, viz., that brought in by the sper- 

 matozoon. 



The field represented by class A has naturally been well 

 explored by cytologists, for in spermatogenesis the odd 

 chromosome was discovered, and there it is expected; 

 moreover the study of spermatogenesis is attended with 

 less difficulty than oogenesis. Hence comparatively few 

 observers have paid any attention to the behavior of the 

 chromosomes in the maturation of the egg, and cyto- 

 logical evidence of the occurrence of possibly dimorphic 

 eggs in the second class of cases is lacking, though Bal- 

 zer's 1908 observations on oogenesis in the sea-urchin, 

 mentioned by Wilson, 1909 b, indicate that something 

 may be done along this line. 



The cytological evidence bearing upon the Lepidoptera, 

 so far as it goes, however, indicates that the male is 

 morphologically homozygous. There is no dimorphism 

 of spermatozoa, the same number of chromosomes being 

 found in all the spermatids. There is, however, a hetero- 

 chromosome, interpreted by the various observers as a 

 pair of .equal idiochromosomes, associated with the plas- 

 mosome in the growth period. According to Dederer, 

 1907, and Cook, 1910, it ultimately becomes indistinguish- 

 able from the other chromosomes, though in the butterfly 

 and the moth examined by Stevens, 1906, its large size 

 made it visible through the maturation mitoses, in both of 

 which it divides into equal parts. Thus, in the seven 

 moths and one butterfly (Euvanessa antiopa) examined 

 by these observers, there is cytological evidence, if the 

 chromosome theory of sex determination be assumed, 

 that the male is homozygous. Unfortunately we have 

 no exact information, so far as I am aware, as to oogene- 

 sis in butterflies. If it should be shown that in Lepidop- 

 tera there is a visible dimorphism of ova as regards the 

 number of chromosomes, the cytological interpretation 

 of sex determination would receive an interesting and 

 important confirmation. 



If such visible dimorphism should be discovered, it 

 would be most interesting to see what bearing it has, if 

 any, upon the question whether the homozygous male is 



