ARTICLES 441 



Early in the investigation Wilson found, in Hemipteran 

 insects, that there was a second case in which the X chromosome 

 of the male is accompanied by a mate of a different type, 

 usually much smaller than X and termed the Y chromosome. 

 In this case, X and Y conjugate during maturation so that half 

 the resulting gametes contain X and half contain Y. The 

 X class are female-producing, the Y class male-producing. 

 (Fig. 2.) 



These two types are connected by a series of intermediate 

 conditions, which in the main are comparable with the two 

 simple cases given above. 



This postulate of the fertilisation of an egg by two different 

 classes of spermatozoa has received proof from observations 

 on the nematode, Ancyrocanthus, where the chromosomes 

 remain visible in the mature spermatozoa and can be counted 

 in the living object. Mulsow traced both types of spermatozoa 

 into the egg and was able to convince himself of the sexual 

 differences in the germ nuclei. Further, an X chromosome has 

 recently been described in a Liverwort. 



It is impossible to go into all the evidence for the conclusion 

 that the X chromosomes are intimately connected with sex- 

 production ; suffice it to say that the peculiar type of inheritance 

 known as sex-limited fits so beautifully with the sex-chromosome 

 hypothesis as to render it difficult to accept any other view. 

 This aspect of the subject is admirably treated by Morgan in 

 his latest book. The Mechanism of Heredity. 



Finally, mention may be made of the surprising position 

 of the Lepidoptera and Birds as regards sex-production. 



For some years it has been known, from a study of sex- 

 limited inheritance, that in these two classes of animals the usual 

 condition was reversed, leading to the conclusion that there were 

 male and female determining eggs, not sperms, as in other classes 

 of animals. 



Cytological investigations by Seiler ^ and Doncaster * have 

 shown that, as regards the Lepidoptera, there are females pro- 

 ducing two types of eggs. Thus we must conclude that, in the 

 majority of animals, the spermatozoa are different, and the 

 eggs alike, but that in the Lepidoptera, at least, it is the sperms 

 that are alike and the eggs which differ, as regards the 

 accessory chromosomes. 



To account for the mode of action of the accessory chromo- 

 somes in sex determination two hypotheses have been 

 put forward — the qualitative and the quantitative. 



In the former it was assumed that the female was heterozy- 



1 Seller, ]., Zool. Anzeiger, 41, 1913, p. 246; Zeitschr. f. indukt. Abst., 



18, 1917. P- 81. ~ 



2 Doncaster, L., Journ. Genetics, 4, 1914, p. i. 



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