CHROMOSOMES AND MITOCHONDRIA 249 



part of their substance into the cytoplasm. This 

 phenomenon has been used as an argument in 

 favor of the theory of nuclear control of cellular 

 activities. Two special cases of chromatin-diminu- 

 tion are known which differ from the usual process ; 

 these occur in Ascaris and Miastor as described and 

 figured in Chapters III and VI. In these animals a 

 large portion of the chromosomes of certain nuclei 

 is cast out into the cytoplasm, whereas all of the 

 chromatin is retained by others ; the latter with a 

 complete amount become the nuclei of the germ cells, 

 the rest with a reduced amount are present in all of the 

 somatic cells. 



During the cellular divisions which result in the 

 multiplication of the somatic cells and of the primor- 

 dial germ cells the chromosomes appear at each 

 mitosis in their normal number and are apparently 

 divided equally between the daughter cells. There 

 are, however, certain variations in both the somatic 

 and germinal mitoses. In the somatic cells only 

 one-half the normal number may appear; thus in 

 the snail, Helix pomatia, the number may be twenty- 

 four instead of the usual forty-eight. There is 

 reason to believe that each of these twenty-four really 

 consists of two single (univalent) chromosomes, 

 and may therefore be considered bivalent. Even 

 a further reduction in number by the association of 

 univalent chromosomes has been recorded, in which 

 case the combined chromosomes are said to be pluri- 

 valent. Other variations in the number of chromo- 

 somes, which occur during the maturation of the 

 germ cells, will be referred to later. 



