636 PRACTICAL PROBLEMS 



This view of the case made it appear that the female cells cor- 

 respond most closely with those spermatozoa which are destitute 

 of the accessory, and here the matter stood until Montgomery 

 (1904), Gross (1904), and Wallace (1905) discovered that, in cer- 

 tain species at least, the ovum has the same number of chromo- 

 somes as the spermatozoa with the accessory. These experimenters 

 came to the conclusion that " only one of the two classes of sper- 

 matozoa is functional, namely, that in which the heterotropic (ac- 

 cessory) chromosome is present. Those of the other class were 

 assumed to degenerate, after the fashion of polar bodies." l 



Wilson (1906), shows that "the sexes in hemipters of this 

 type do in fact show a constant difference in the number of 

 chromosomes." 2 He has determined, in at least four genera, 

 that the number of chromosomes in the female cell corresponds 

 with the larger number in the male cell; in other words, that the 

 "accessory chromosome," though present in but half the sper- 

 matozoa, is found in all female germ cells. This being true, the 

 " remarkable " spermatozoa are not those with the accessory, 

 but, on the contrary, those without it, and they are to be 

 regarded as in some sense deficient. 



Wilson shows conclusively the opposite of Gross' and Wallace's 

 hypothesis, namely, that when a female cell unites with a sper- 

 matozoon destitute of the accessory chromosome, then the 

 accessory of the ovum finds no mate and a male develops ; and 

 that, on the other hand, if the ovum happens to be fertilized by 

 one of the spermatozoa provided with an accessory, then each 

 accessory finds its mate, there is then no solitary accessory, and 

 a female results. 



Extending his experiments, Wilson finds two kinds of acces- 

 sory chromosomes, the one already described, which is smaller 

 than the ordinary chromosomes, and another which is larger. In 

 this connection it should be remarked that his investigations 

 show great differences in size among the chromosomes generally, 

 but that the "accessory" can readily be detected, whether 

 larger or smaller than the others, and that all chromosomes, 

 large or small, except the accessory, can readily be assigned 

 in pairs under the microscope. 



^ Journal of Experimental Zoology, III, No. I, p. 2. ' 2 Ibid. 



