302 GERM-CELL CYCLE IN ANIMALS 



constitute an important part of the germ-plasm or 

 play a minor role 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 (Fi) 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 (F 2 ) generation were yellow 



