24 PETER OKKELBERG 



Waldeyer in 1870. He found some large spherical cells in the 

 coelomic epithelium on each side of the dorsal mesentery in the 

 early chick embryo, and supposing that they were young stages 

 of eggs he called them ' Ureier' ; the epithelium in which they were 

 found he called 'Keimepithel' (germinal epithelium). In 1875 

 Semper found that both ova and spermatozoa were derived from 

 the so-called Ureier. Waldeyer and his followers believed that 

 these cells were derived directly from the cells of the germinal 

 epithelium, and this idea is held by a few investigators at the 

 present time. 



Since the development of the theory of 'early segregation' by 

 Nussbaum ('80) many investigators have worked on the origin 

 of the germ cells in vertebrates. It has been found in most 

 cases in which the early history of the cells has been traced that 

 they do not originate in the coelomic epithelium, among the 

 cells of which they are later found, but that they attain this 

 position after a migration from other parts of the embryo 

 (Woods, Allen, Dodds, King, Witschi, and others). These 

 same investigators have found that the germ cells in very early 

 stages are usually located in the entoderm, from which they 

 migrate to their definitive position in the coelomic epithelium. 

 When first found, they are large yolk-bearing cells of the ento- 

 derm and distinguished from the entoderm cells principally by 

 their location. 



From numerous investigations on the subject there seems to be 

 no doubt about the existence of the so-called primordial germ cells 

 which are segregated very early in the development of the 

 embryo, but whether or not all, some, or any of the definitive 

 germ cells are derived from these is a question about which 

 there is very little agreement. Rubaschkin ('09, '12) and others 

 hold that the definitive reproductive cells in mammals are derived 

 exclusively from primordial germ cells. Firket ('14) thinks it is 

 possible that a few of the oogonia may be derived from the pri- 

 mordial germ cells, but that most of them are derived from 

 certain cells in the germinal epithelium, which he calls 'gono- 

 cytes secondaire.' Kuschakewitsch ('10) believes that in Rana 

 esculenta the oogonia are derived from cells in the germinal 



