300 GERM-CELL CYCLE IN ANIMALS 



It is becoming more and more evident, however, 

 that the cytoplasm cannot be entirely excluded. As 

 noted in Chapter IX, the mitochondria appear to be 

 constant cell elements and may actually constitute 

 a part of the essential hereditary substance. Even 

 if these particular cytoplasmic bodies do not repre- 

 sent germ-plasm, still, as pointed out by Guyer (1911) 

 and others, cytoplasm as well as nuclear material is 

 necessary to explain the phenomena which we call 

 heredity. It was shown in Chapter I that the most 

 important primary constituents of protoplasm are 

 the proteins, and the idea is rapidly becoming general 

 that the mechanism of heredity consists of (1) fun- 

 damental species substances, probably mainly pro- 

 tein in nature, together with (2) equally specific 

 enzymic substances which regulate the sequences of 

 the various chemical and physical processes incident 

 to development (Guyer, 1911, p. 299). The chro- 

 mosomes have been suggested as enzymatic in 

 nature (Montgomery, 1910), but enzymes are sup- 

 posed merely to accelerate reaction already initiated, 

 and hence the substrate must be of as great importance 

 as the enzymes which work upon it. But the sub- 

 strates must be extremely numerous to supply each 

 species with its specific proteins. That there are 

 enough configurational differences in corresponding 

 protein molecules to supply the number for the 

 thousands of animal species is certain, since some 

 comparatively simple proteins may possess thousands 

 of millions of stereoisomers. Thus the study of 

 heredity substance involves primarily a knowledge 



