Il8 HOW ANIMALS DEVELOP 



One can only make a rigid distinction of this kind 

 between the temporary body and the immortal 

 germ-cells if one can show that the germ-cells are 

 never really a true part of the body but are always 

 fundamentally separate from it. Now in many cases 

 it is probably true that the germ-cells never do 

 belong to the body in any sense except that of just 

 being inside it. We find, for instance, that at the very 

 beginning of development there may be a particular 

 part of the tgg which looks unlike the rest and is set 

 apart to become the germ-cells ; in such cases there 

 is, in fact, an organ-forming substance for germ-cells, 

 and the rigid distinction between body and germ- 

 cells may be plausible. But often no such substance 

 can be found, and the germ-cells are probably 

 formed in the same way as the other differentiated 

 cells in the body, that is, in response to some 

 organizer. In this sort of embryo the germ-cells 

 really act like a part of the body and not as though 

 they were inhabiting it and did not belong to it. 



Then again, it is not only the germ-cells which are 

 potentially immortal. All young embryonic cells are, 

 and probably even the highly specialized cells in the 

 different organs of an adult animal could remain 

 alive almost indefinitely even although they might 

 not be able to grow and divide. It is the animal as a 

 whole which dies, not the individual cells of which 

 it is made. The immortality of the cells can be 

 realized by removing them from the body and 

 growing them in a nutritive medium in culture, like 

 the embryos mentioned in Chapter m. Anyone 



