CELLS LINING THE PERITONEAL CAVITY 421 
up their embryonic character and have not been differentiated 
into any especial somatic structures. Eigenmann (’91), Beard 
(04), and Hoffmann (’92) have also supported this view. Swift 
(15) derives the primordial germ-cells from a particular part of 
the germ-wall entoderm at the edge of the area pellucida during 
the primitive-streak stage. When this area becomes vascularized 
by the extension of the mesoderm, these cells gain entrance to 
the circulation and settle out in the region of the developing 
gonad. He agrees that the medullary cords are formed from 
downgrowths of the germinal epithelium, but considers the cords 
of the second proliferation as formed by rapidly proliferating, 
primordial germ-cells or oogonia. Finally, the oogonia form 
the definitive ova, while the cells derived from the germinal 
epithelium present in the cortical cords become follicular 
epithelium. 
A third group of workers have assumed that there are two 
sources for the definitive ova, one the primordial germ-cells 
which furnish a few ova, while a second generation of germ-cells 
develop from the germinal epithelium. This view has been 
supported by Felix (06), Allen (’04), Dustin (’07), and particu- 
larly by Firket. Firket (14, ’20 b) has described the early 
stages in the chick and rat, and in both species he finds the total 
number of the primordial germ-cells far too few to permit of 
their being considered the sole source of definitive ova, so that 
he is forced to assume an additional development of ova from 
the germinal epithelium. Using his two species as types of the 
bird and mammal, he suggests that this process may be a phy- 
logenetic recession. 
The discussion of the origin of the definitive ova from the 
germinal epithelium deals almost entirely with changes which 
take place during embryonic life, and hence, in mammals at 
least, would be most difficult to examine by the vital staining 
technique. However, it is well known that in some species there 
is formation of ova after birth, and in these it seems that the 
vital staining of the germinal epithelium might prove a useful 
adjunct to the study of the question regarding the origin of the 
definitive ova. My experiments, being confined so far to the 
