238 PRINCIPLES OF ANIMAL BIOLOGY 



cells may and do produce body cells, but except perhaps in very low 

 nietazoa it is improbable that body cells ever produce germ cells.* 



This independence of the germ cells need not be insisted on in most 

 ordinary relations of animals. Indeed, in a physiological sense it ma^^ 

 tend to obscure certain fundamental facts, such as the dependence of the 

 germ cells upon the body for nutrition. The body is the environment 

 of the germ cells. But in some phases of genetics and evolution the 

 distinctness of germ and body cells is a useful conception. 



References 



Bailey, F. R., and A. M. Miller. Textbook of Embryology, Chapters I-VI. 

 Hegner, R. W. The Germ Cell Cycle in Animals, Chapters I and II. 

 Holmes, S. J. The Biology of the Frog, Chapter V. 

 Kellicott, W. E. a Textbook of General Embryology. 

 Kellicott, W. E. Outlines of Chordate Development. 

 Morgan, T. H. The Development of the Frog's Egg. 



Prentiss, C. W., and L. B. Arey. A Laboratory Manual and Textbook of Embry- 

 ology, Chapters I and II. 



1 Much of the conception of the independence and distinctness of the germ cells 

 and somatic cells is inapplicable to plants, in which germ cells are not formed until 

 late in the life of the individual, and in which germ cells may, under suitable circum- 

 stances, originate from many parts of the body. 



