302 GERM-CELL CYCLE IN ANIMALS 
constitute an important part of the germ-plasm or 
play a minor réle in heredity is still uncertain. 
Modern cytological studies and the results of ex- 
perimental breeding both help to solve the prob- 
lems of the combination and subsequent distribution 
of the determiners or factors within the germ-plasm. 
In fact, it has been maintained by certain geneticists 
that “The modern study of heredity has proven 
itself to be an instrument even more subtle in the 
analysis of the materials of the germ cells than actual 
observations on the germ cells themselves ” (Morgan, 
1913, p.v). Those who do not wish to commit 
themselves as to the physical or chemical nature of 
the germ-plasm are content to speak of determiners, 
factors, or genes without connecting them with any 
particular substances. The behavior of the chro- 
mosomes, however, enables us to explain so many of 
the facts of heredity that, as stated above, these 
bodies are generally considered to constitute the 
essential hereditary substance. 
The study of heredity was wonderfully stimulated 
by the recognition in 1900 by Correns, Von Tscher- 
mak, and de Vries of the results of Mendel’s (1866) 
investigations on plants. One of the simplest of 
Mendel’s experiments is that which he performed 
with differently colored peas (Fig. 81). A pea bear- 
ing green seeds was crossed with a pea bearing yellow 
seeds. The first (F1) generation of peas resulting 
from this cross all bore yellow seeds. When the in- 
dividual plants of this generation were inbred, three- 
fourths of the resulting (Ff) generation were yellow 
