INHERITANCE 



291 



alone have descendents living on in successive generations. 

 Being protected within the body of a multicellular organism 

 and having no nutritive functions to perform, they are 

 removed from direct contact with environment, and remain 

 unspecialized with reference thereto. The germ cells are 

 developed from the egg along with the body cells, but are set 

 apart therefrom, sooner or later in the course of differentia- 

 tion. Soon the body cells invest the germ cells with a cover- 

 ing in which they are sheltered and nourished during all 

 their subsequent development. This general relation 

 between germ plasm and body plasm is diagrammatically 

 set forth in figure 174. 



FIG. 174. Diagram of the relation between germ plasm and 

 body plasm, s, body plasm (soma), egg and sperm shown 

 below, and zygote (circle inclosing dot) beyond; s, s, s, the line 

 of succession; t, the line of descent. 



It may well be, therefore, that parent and offspring 

 resemble each other because both are developed from the 

 same stock of germ plasm. 



Every organism begins life as a single cell. It behooves 

 us, therefore, to look a little more closely into the structure 

 of the cell. Since the fertilized egg may develop into the 

 complete individual without further parental influence, that 

 new individual must be potentially present, and also the 

 mechanism whereby its parts are wrought out. To the egg 

 cell let us go, therefore, to learn further of the nature of this 

 mechanism. Our task will be easier if we examine the 

 minute transparent, nearly yolkless eggs of such simple 

 marine organisms as sea urchins and starfishes, which if 

 placed alive in sea water under the microscope will go on 

 developing, the divisions succeeding each other in quick 



