26 GEOKGE T. HARGITT 



thereby definitely established, though this is clearly dependent 

 upon the proper stimulus for its exhibition. When we take into 

 consideration, also, the observations upon the origin of germ 

 cells from tissue cells; the observations of Child upon the de- 

 differentiation of cells in a great variety of animals and their 

 later differentiation into a different sort of cell; the observations 

 upon the formation of embryonic masses from which new 

 structures develop in regenerating worms and salamanders; it 

 would seem as though the germ-plasm theory was the very one 

 of all theories least capable of accounting for the facts. 



3. Evidence from other phyla 



Such phyla as the round worms and arthropods give the 

 strongest evidence of early segregation of germ cells and the best 

 support of the germ plasm theory. This view is not universally 

 accepted, however, and the opposing opinions are worthy of con- 

 sideration. For instance. Child ('15) states that it is not known 

 whether the primordial germ cells of Ascaris produce only germ 

 cells or the reproductive organs as well. If the latter be the 

 case, ''the germ path of early cleavage has not resulted in the 

 segregation of germ plasm from the soma, but merely in the 

 segregation of different organs," since the walls of the repro- 

 ductive organs are not germ plasm. The same author points to 

 the fact that in no case is a segregation of germ plasm and soma 

 known to take place at the first cleavage, as the theory requires. 

 He believes, even in these phyla, the theory is unproved, and is 

 not in accord with many facts. 



In many animals the germ cells are produced periodically at 

 the breeding season, and at no other period is it possible to 

 recognize germ cells, or even reproductive organs. In these 

 cases the germ cells obviously arise from the tissue cells ; it 

 does not answer to claim an invisible germ plasm in the tissue 

 cells, since this is not capable of investigation and evades the 

 question. Other animals are produced asexually and at a later 

 period develop reproductive organs; the germ cells to all appear- 

 ances, in such cases, come from the more or less differentiated 

 cells of the region involved in the formation of these organs. 



