164 GENERAL ZOOLOGY 



Though a part of the body, the germ cells remain distinct from 

 body cells, and when favorable conditions arise for the union of 

 ovum with sperm, they proceed forthwith to display their remark- 

 able developmental energy. Any other cell, such as muscle or 

 liver cell, does not have this reproductive power, yet each, like 

 the germ cell, has its origin in the fertilized egg. In becoming 

 specialized body cells, it seems that the tissue cells lose the repro- 

 ductive capacity, which must have been latent in them at the 

 beginning. Germ cells, on the other hand, retain it because they 

 do not differentiate into tissue cells. It would follow that once 

 a cell has developed into a tissue cell, it can never afterward 

 function as a germ cell. This is true in most animals, there 

 being some apparent exceptions among invertebrates like the 

 Coelenterata, where the germ cells develop from time to time from 

 differentiated endoderm or ectoderm. Obviously, in such cases 

 differentiation of body tissues even in the adult does not proceed 

 so far that a return to an unspecialized state is impossible. 



