310 GENERAL EMBRYOLOGY 



giving a somatic group of twenty-one; or the egg with its 

 eleven chromosomes may be fertilized by a sperm with eleven, 

 giving a somatic group of twenty-two. And since there are 

 equal numbers of ten- and eleven-chromosome spermatozoa, 

 there will be approximately equal numbers of zygotes with 

 twenty-one and twenty-two chromosomes. These relations 

 are shown in diagrammatic form in Figs. 143, 144. 



Since this numerical difference between the somatic chromo- 

 some groups is constantly associated with sex-difference, males 

 possessing twenty-one, females twenty-two chromosomes, it 

 may be said that the presence of the idiochromosome is in 



Mature ova. All Spermatozoa, 



of one class Two classes 



Male 



FIG. 144. Diagram of the relations of chromosome number and sex during 

 fertilization in Anasa. The essentials are the same in cases where X is a multiple 

 element, or where it is paired with a Y-element. 



some way connected with the determination of the female, or 

 the lack of it the male, sex. 



Since Paulmier's description of these events in Anasa, in 

 1899, a great many similar instances have come to light, and 

 more recently quite a variety of conditions related to this but 

 more or less dissimilar in details, have become known. The 

 X-chromosorne or idiochromosome has been described under 

 many different names such as accessory chromosome, allosome, 

 heterotropic chromosome, heterochromosome, monosome, etc. Such 

 an unequal distribution of the chromosomes was first observed 

 by Henking in 1890, and in 1902 McClung described similar 

 processes in several of the locusts and grasshoppers (Orthop- 

 tera), and first suggested the possible relation between this 



