392 THE DEVELOPMENT OF THE CHICK 
and absence of a basement membrane from the adjacent peri- 
toneum (Fig. 217). The germinal epithelium les 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 Hzematopus, 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 jater stages (Semon). The embryos of birds are not well 
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 
