ORIGIN OF PRIMORDIAL GERM CELLS 485 



The supporters of the third theory, that of early segregation, 

 agree with the exponents of the other two, that the primordial 

 germ-cells are at one time in their history found in the germinal 

 epithelium. They do not, however, believe that they arise in 

 situ, nor from the gonotome, but that they have a much longer 

 history dating back to an early embryonic stage. It may be 

 possible, at some future time, to trace the primordial germ-cells 

 back to the very early divisions of the segmenting egg, if not to 

 the fertilized egg itself. In vertebrates, the work of Eigenmann 

 ('97), demonstrated an origin of primordial germ-cells within five 

 divisions of the fertiUzed egg; that is, he found that the young- 

 est germ-cells possessed a size equivalent to blastomeres of the 

 fifth cleavage. If this last is correct, that the germ-cells have 

 an independent existence from a very early period, and are not 

 differentiated from somatic cells, then the idea of germ-plasm 

 continuity has received valuable support. This idea of early and 

 extraregional origin is borne out by all recent research, and is be- 

 coming more firmly established with each succeeding year. 



The names of Hoffmann ('93) Eigenmann ('97), Nussbaum ('01) 

 and Beard ('04) are associated with the early years of this early 

 segregation theory of germ-cell origin. 



Hoffman ('93) was the first to bring forward any evidence 

 which cast doubt on the theory of origin of primordial germ- 

 cells from the germinal epithelium. He employed in his work 

 twelve species of birds, in three of which, Haematopus ostralegus, 

 Sterna paradisea, and Gallinula chloropus, there was sufficient 

 evidence brought out to prove that some, if not all the primor- 

 dial germ-cells, did not originate in the modified coelomic epi- 

 thelium. In the three species mentioned above, he found at the 

 proper time, numbers of primordial ova in the germinal epithe- 

 lium. But, in addition he found cells— supposedlj^ primordial ova, 

 because of their resemblance to those found later in the germinal 

 epithelium — in embryos of 23 somites. An embryo of 23 so- 

 mites does not possess the so-called germinal epithelium, the coe- 

 lomic epithelium over the Wolffian body not having been modi- 

 fied at this age, yet in these he found primordial germ-cells far 

 removed from the site of the future sex-gland, in the splanchnic 



