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 beof 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 
