Internal Factors of Sex Determination 403 



of spermatozoa, presumably both functional, there may be a 

 connection between the two conditions. This tacitly assumes 

 that all of the eggs are alike, which was apparently taken for 

 granted by McClung, but not examined. Since the accessory 

 is found in one half of the spermatozoa, McClung conjectured 

 that those spermatozoa that contained the accessory produce 

 males, presumably because of the seeming necessity for the new 

 males to have an additional chromosome, the accessory. This 

 argument proved invalid in the Hght of the observation of 

 Wilson, that the female and not the male contains an additional 

 chromosome, and of Stevens that the female of Tenebrio molitor 

 contains the large chromosome (the homologue of the acces- 

 sory) and the male the small chromosome. 



In order to show how this process works itself out, let us take 

 the case of Anasa tristis, in which there are 21 chromosomes in 

 the male somatic cells and 22 in the female. There will be 

 II chromosomes in half of the spermatozoa and 10 in the other 

 half. All of the eggs may be supposed to contain 11 chromo- 

 somes. When a lo-chromosome spermatozoon fertilizes an egg, 

 there is formed a male with 21 chromosomes, w^hose sperm will 

 again contain 10 and 11 chromosomes; when an ii-chromosome 

 spermatozoon fertilizes an egg, there will be 22 chromosomes — 

 the new individual will contain 22, and its eggs will later all 

 contain 11 chromosomes. The accompanying scheme shows 

 this result graphically : — 



Spermatozoon Egg Male Germ-cells 



10 > II = 21 > 10 or II 



or or Female 



II > II = 22 > II or II 



The evidence that the eggs are all alike and contain the larger 

 number {i.e. 11 in the case just cited) is indirect, but apparently 

 conclusive. The chromosomes of the polar spindles have not 

 been counted, but the somatic cells contain 24 chromosomes, 

 as well as the early germ-cells before the pairing of the chromo- 

 somes. Moreover, in those cases in which the accessory of the 



