CELL DIVISION AND GAMETOGENESIS 231 



in which connection they will be referred to again in a later 

 chapter. 



Sex Chromosomes. — In the hypothetical example used to 

 illustrate mitosis and gametogenesis, the zygotic number of 

 chromosomes was assumed to be 8. Though constant for any 



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Fig. 142. — Chromosomes of the squash bug, Anasa tristis. a, polar view of 

 equatorial plate, first spermatocyte; b, side view of anaphase, second spermato- 

 cyte; the heterochromosome (h) passes undivided to one pole of spindle; c and 

 d show the results of the second spermatocyte division, ten chromosomes going 

 to one cell (c) and eleven to the other (d), so that two classes of spermatozoa 

 result; e, polar view of spermatogonium showing twenty-one chromosomes; 

 /, diploid chromosome complex of male arranged in pairs according to size, and 

 forming a biparental series; g, polar view of equatorial plate of oogonium show- 

 ing twenty-two chromosomes; h, diploid complex of female, paired according to 

 size. (From Wilson, Journal of Experimental Zoology.) 



given species, the number may actually be from 2 to 100 or jnore. 

 Thus in the nematode worm Ascaris megalocephala univalens the 

 zygotic number is 2; in the frog, Rana pipiens, 26; in man, 48; 

 etc. In some cases the number may be different in the male and 

 female of the same species, an example of which may now be 

 considered. Anasa tristis is the common squash bug. In this 

 form the zygotic number of the female is 22, while that of the 



