484 CHARLES H. SWIFT 



would be much more difficult of demonstration than when col- 

 lected into a relatively circumscribed area like the germinal epithe- 

 lium or developing gonad. In the older embryos, however, they 

 were able to see the typical primordial germ-cells in the modified 

 coelomic epithelium, and so naturally thought that they arose 

 by a series of transformations from the epithelial cells. Semon 

 ('87) claimed to have seen a number of cells undergoing this 

 change of form, but since that time no other investigator has 

 ever been able to see any intermediate stages between the cells 

 of the germinal epithelium and the primordial germ-cells. D'Hol- 

 lander ('04) described in the chick embryo of ten days, however, 

 the production of oogonia from a superficial indifferent layer of 

 cells — another term for germinal epithelium. According to d'Hol- 

 lander, the germinal epithelium gives rise to epithelial buds, which 

 grow down into the ovarian stroma. From these epithelial, germi- 

 native buds, by a process of differentiation, the oogonia and 

 primordial foUicular cells arise. This origin of oogonia at such 

 a late period and from the germinal epithelium must be second- 

 ary to the production of primordial germ-cells, which, according 

 to all authorities, make their appearance in the gonad at a much 

 earlier period of development than ten days. 



Another view as to the origin of primordial germ-cells may be 

 called the gonotome theory. The supporters of this theory agree 

 with the advocates of the germinal epithelium origin in so far 

 that the primordial germ-cells are found at one time in the modi- 

 fied coelomic epithelium. They, however, claim that the pri- 

 mordial germ-cells do not arise in the germinal epithelium, but 

 from a portion of the segmental mesoderm, and reach the epithe- 

 lium at a later period either by migration or by tissue growth. 

 Riickert ('88) advanced this idea, and called that portion of the 

 segmental mesoderm, from which he supposed they took origin, 

 the gonotome. That certain of the primordial germ-cells are, at 

 times, found in the region of the segmental mesoderm there can 

 be no doubt, but, their occurrence there can be explained in an- 

 other and better way than by supposing that they take origin 

 there. This theory has had few advocates, and is of importance 

 now, mainly because of the historical interest attached to it. 



