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. 
