ORIGIN OP THE INDIVIDUAL 



223 



C. ORIGIN OF THE GERM CELLS 



Among the Vertebrates, as we know, 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 



I 



B' 



I 

 t 

 $& 



C' 



FIG. 122. Regeneration of a Flatworm (Planaria maculata). A, normal 

 worm; cut across at line indicated. B, B', and C, C', regeneration of an- 

 terior and posterior parts of A to form complete worms. D, piece cut from a 

 worm; D l , Z> 2 , D 3 , Z> 4 , successive stages in the regeneration of D. E, 'head' from 

 which rest of animal has been cut off. E 1 , E 2 , E s , successive stages in the re- 

 generation by E of a complete body. F, similar experiment to E, but a new 

 'head' in reversed position is regenerated instead of a body, F l . (From Hegner, 

 after Morgan.) 



primordial germ cells do not arise as such by division in the 

 tissues which during development form the ovaries and testes. 

 Just when the germ cells are set aside in Vertebrates is un- 

 certain, but it would seem to occur very early in embryonic 

 life, perhaps during the cleavage of the egg. Then by 

 shif tings 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 



