380 KING. [Vor. XIX. 
by follicle cells as in an earlier period. At a slightly later 
stage of development (Fig. 16), the central part of the ovary 
is no longer completely filled with peritoneal cells, but it con- 
tains a number of intercellular spaces which later unite to form 
one large cavity (Fig. 17). The central cavity in the ovary 
of Bufo is not, therefore, a portion of the general body-cavity 
which is brought into the ovary by a fold of peritoneal epithe- 
lium as Hoffman has claimed is the case in Triton and in 
various other amphibians, but it is the result of a fusion of 
the many intercellular spaces which-are produced by the rapid 
increase in the size of the ovary. In Fig. 16 is shown the 
beginning of the formation of the outer ovarian wall. At 
the upper part of the ovary a number of peritoneal cells are 
found with their nuclei flattened against the outer surface of 
the germ-cells. The outlines of these cells become obliterated 
and their cytoplasm forms a continuous layer over the 
oogonia. At a slightly later stage (Fig. 17), many of the 
peritoneal cells in the interior of the ovary become arranged 
along the inner side of the oogonia to form the inner wall 
of the ovary. In the young female as well as in the adult, 
the ova develop between the two ovarian walls. 
The small cells with deeply staining nuclei which are so 
conspicuous in the ovary at the stages of Figs. 15-17 have 
been called by various observers mesenchyme cells, peritoneal 
cells, and follicle cells; while Bouin considers them to be 
“petites cellules germinatives.”” In Bufo these cells are found 
with the primordial germ-cells when the latter are first sepa- 
rated from the endoderm at the stage of Fig. 3, and from their 
general characteristics they are doubtless to be classed as 
mesodermal cells. Occasionally these cells are found dividing 
mitotically (Fig. 15, R); but division figures in them are 
rare as compared with those that are found in the germ-cells. 
The number of these cells increases enormously as the ovary 
enlarges; and, since there is no evidence that they divide 
amitotically, it is probable that there is a continuous migration 
of cells from the mesentery through the ovary pedicle into 
the ovary. Many of these cells later become the follicle cells 
which are found-around the egg as long as it remains in the 
