A CRITICAL EXAMINATION 5 



the several gametes must ^be determined, proxi- 

 mately at least, even if not initially, during the 

 process of gamete formation. 



C. Somatogenesis. (Development and differentia- 

 tion.) 



The adult offspring, in whose adult characters we 

 perceive a resemblance to the parents, is the result 

 of a long and complicated process of development 

 and growth from a single cell. This cell is itself 

 a composite structure produced by the fusion of 

 two cells, one derived from each of the parents. 

 From the standpoint of heredity the most striking 

 thing about the fertilized germ cell is that it carries 

 the potentiality of producing a higher degree of 

 differentiation in the individual which develops 

 from it, than it exhibits in its own structure. 

 The existence of such potentiality is demonstrated 

 by the specificity of the developmental behavior 

 of the fertilized egg. Under no circumstances 

 does a hen's egg ever develop into a turkey. 



While the germ cell is distinguished from other 

 cells by its potentiality, when separated from other 

 cells of the body and appropriately stimulated, 

 to develop and differentiate, yet it must be recog- 

 nized that this is by no means a unique property 

 of germ cells. Early studies on regeneration 

 and the development of isolated blastomeres, 

 and the recent experiments on the culture in vitro 

 of more highly differentiated somatic cells, show 

 clearly enough that this property is common in 



