PORIFERA, CCELENTERATA, VERTEBRATA 99 



not strange that he should have come to the con- 

 clusion that the primordial ova arise from the 

 epithelial cells of the genital ridge among which they 

 were observed. Although this theory was accepted 

 by most embryologists, it has gradually been aban- 

 doned until now it has very few supporters. 



The gonotome theory resulted from the studies of 

 Ruckert (1888) and Van Wijhe (1889). The germ 

 cells appeared to these investigators to arise in a 

 part of the segmental mesoblast of the embryo to 

 which the latter applied the term ' gonotome.' 

 From the gonotome they become embedded in the 

 peritoneum. Thus the same cells are recognized as 

 germ cells by the adherents of both theories, but a 

 difference exists regarding their origin. 



The theory of early segregation has become the 

 most prevalent view of the origin of the germ cells 

 of vertebrates, although there are many who still 

 hold one of the other hypotheses. According to 

 this theory the germ cells are set aside during the 

 early embryonic stages before definite germ layers 

 are formed, and they later arrive at the germinal 

 ridge either by their own migration or by changes in 

 the position of the tissues during development. 

 The germinal epithelium theories have little if any 

 evidence in their favor, since no one has actually ob- 

 served a transformation of peritoneal or mesoblast 

 cells into germ cells. On the other hand, there is an 

 abundance of proof that these cells migrate from 

 some distance into the position of the sex glands. 



According to Dustin (1907), Firket (1914) and 



