392 THE DEVELOPMENT OF THE CHICK 



from the adjacent peritoneum (Fig. 217). The germinal epithe- 

 lium 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 the primordial germ-cells. The 

 latter are typically round, and several times as large as the 

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

 but contains persistent yolk granules and a large attraction 

 sphere, and the nucleus contains one or two nucleoli; they 

 are sharply distinguishable from the peritoneal cells, and they 

 may be traced through a continuous series of later develop- 

 mental stages into the ova and spermatozoa. The origin of 

 these primordial germ-cells 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. 

 The former view was due to Waldeyer, and was supported by 

 observations of cells intermediate in structure between the pri- 

 mordial germ-cells and cells of the peritoneum (e.g. by Semon). 

 These observations have, however, been shown to be erroneous. 

 The second view has been demonstrated for a considerable number 

 of vertebrates; and quite recently Swift has shown that the 

 primordial germ-cells of the chick arise from the germ-wall at the 

 anterior margin of the pellucid area in a late stage of the primitive 

 streak; that they later enter the blood stream and are carried 

 into the embryo; some, which reach various inappropriate posi- 

 tions, degenerate; but others leaving the blood near the base of 

 the mesentery reach the germinal epithelium by migration. The 

 independent and early origin of germ-cells has an obvious 

 bearing on the theory of the continuity of the germ-plasm of 

 Weismann. 



