392 THE DEVELOPMENT OF THE CHICK 



and absence of a basement membrane from the adjacent peri- 

 toneum (Fig. 217). The germinal epithelium lies between the 

 base of the mesentery and the mesonephros at first, but as the 

 latter grows and projects into the body-cavity the germinal 

 epithelium is drawn on to its median surface. It is difficult to 

 determine its antero-posterior extent in early stages; it begins 

 near the point of origin of the omphalomesenteric arteries, and 

 its posterior termination is indefinite, but it certainly extends 

 over seven or eight somites. 



Two kinds of cells are found in the germinal epithelium, viz., 

 the ordinary peritoneal cells and primitive ova. The latter are 

 typically round, and several times as large as the peritoneal 

 cells (Figs. 226 and 227) ; the cytoplasm is clear and the nucleus 

 contains one or two nucleoli; they are sharply distinguishable 

 from the peritoneal cells in most cases, and they may be traced 

 through a continuous series of later developmental stages into 

 the ova and spermatozoa. The origin of these primitive ova is 

 therefore a matter of considerable interest. 



Two views have been held: (1) that they are derived from 

 the peritoneal cells, and (2) that they have an independent history 

 antecedent to the differentiation of a germinal epithelium, repre- 

 senting in fact undifferentiated embryonic cells that reach the 

 germinal epithelium by migration from their original source. In 

 support of the latter hypothesis the observations of Hoffmann may 

 be cited, who has found cells indistinguishable from primitive ova 

 in embryos of Haematopus, Sterna paradisea, and Gallinula, at 

 a stage of 23 somites, embedded in the mesoderm, mesenchyme, 

 and even the entoderm of the splanchnopleure. (See also Nuss- 

 baum, 1901.) Transitional cells were not found. On the other 

 hand, in the germinal epithelium itself, transitional stages between 

 the primitive ova and the ordinary peritoneal cells are frequent 

 in later stages (Semon). The embryos of birds are not w^ell 

 adapted for the solution of this puzzling question; but in some 

 reptiles and selachia and other vertebrates primitive ova have 

 been traced from a very early stage of the embryo through various 

 migrations to the germinal epithelium. On comparative and 

 theoretical grounds, the view of the independent origin of the 

 primitive ova is preferable; but the origin of some at least from 

 the peritoneal epithelium cannot be disproved for the chick. 

 Two other constituents enter into the composition of the 



