GERM CELLS OF COELENTERATES 27 



In the vertebrates the germ cells appear, as a rule, only after 

 most of the other organs are laid down, and in most cases an 

 early segregation of germ cells has not been proved. A review 

 of the w^ork on vertebrates is given by Hegner ('14) and 

 Kingery ('17), and only a few cases will be mentioned here. 

 Von Winiwarter and Sainmont ('08), from studies upon the cat, 

 describe the degeneration of all the germ cells produced during 

 embryonic development; the definitive eggs arise from the un- 

 differentiated germinal epithelium after birth. Bachman ('14) 

 in Teleosts and Witschi ('14) in Rana temporaria find no evidence 

 of the origin of germ cells from the peritoneum, while v. Beren- 

 berg-Gossler ('14) believes "that one may no longer speak of a 

 germ track in the Sauropsida," and Gatenby ('16), in Rana tem- 

 poraria, observes the majority of germ cells arising from the 

 peritoneum. Kingery ('17), working upon the white mouse, 

 gets results comparable to those of von Winiwarter and Sain- 

 mont in the cat; viz., all germ cells formed during the foetal 

 period degenerate and have nothing to do with the development 

 of the definitive ova. The latter arise from the germinal 

 epithelium after birth and all transitional stages between this 

 germinal epithelium and graafian follicles were observed and the 

 development followed. 



In the vertebrates and some other phyla the evidence seems 

 to be as clearly opposed to a continuity of the germ plasm as it 

 is in the coelenterates. There is, especially in mammals, an 

 increasing amount of evidence that the germ cells arise from 

 more or less differentiated tissue cells at a time approaching the 

 period of sexual maturity. 



4. Evidence from tissue cultures 



AVhile most of the experiments dealing with explanted tissues 

 have to do with growth, movements, and general behavior of 

 the cells, there is some evidence of a de-differentiation of the 

 tissues into a more embryonic condition. There is very little 

 evidence that such cells re-differentiate into cells of a new kind, 

 but this return to an embryonic condition resembles somewhat 

 the despecialization of isolated cells of hydroids and sponges. 



