238 



ANIMAL BIOLOGY 



B. Origin of the Germ Cells 



Among the Vertebrates, as previously described, the germ cells 

 reside during adult life in definite organs, the ovaries and testes, and 

 upon these cells the power of reproduction of the individual is 

 solely dependent. It seems clear, however, that the primary germ 

 cells do not arise as such by division in the tissues which during 



Gametes O 



Prim, Sex Cells 



Fig. 162. — Diagram of the lineage of the body cells and germ cells in a 

 Worm or Mollusc. Lineage of germ cells shown in black, of ectoderm in white, 

 and of endoderm and mesoderm in shaded circles. (From Conklin.) 



development form the ovaries and testes. Just when the germ cells 

 are set aside in Vertebrates is uncertain, but it would seem to occur 

 very early in embryonic life, perhaps during the cleavage of the 

 egg. Then by shiftings of the tissues during growth, and possibly 

 also by amoeboid movements of the germ cells themselves, they 

 finally reach definite positions in the epithelium lining the dorsal 

 wall of the coelom, which becomes an integral part of the gonads 

 as development proceeds. (Fig. 162.) 



With regard to the fate of the primordial germ cells, once they 



