THE CELLULAR BASIS 107 



to define just what such units are or just how 

 they behave, modern students of heredity as- 

 sume their existence. They are now called 

 determiners or factors or genes,, and they are 

 usually thought of as units in the germ cells 

 which condition the characters of the devel- 

 oped organism, and which are in a measure in- 

 dependent of one another; though of course 

 neither they nor any other parts of a cell are 

 really independent in the sense that they can 

 exist apart from one another. They are to be 

 thought of as analogous to chemical radicals 

 which are never independent but exist only in 

 combination with other chemical elements in 

 the form of molecules, and yet preserve their 

 identity in many different combinations. 



If there are inheritance units, such as de- 

 terminers or genes, as practically all students 

 of heredity maintain, they must be contained 

 in the germ cells, and it becomes one of the 

 fundamental problems of biology to find out 

 where and what these units are. But whether 

 we assume the existence of these units or not 

 we know that the germ cells are exceedingly 

 complex, that they contain many visible units 



