1910-11.] Development of Germ Cells in Mammalian Ovary. 161 
Mitosis is seen among these cells and among the first stage of the oocytes. 
The cell nests are smaller near the centre and larger at the periphery. This 
is explained by the ingrowths of the stroma cells being more numerous and 
of greater density in the central portion. In the deeper layers are seen 
numerous cells in the stage of synapsis and in the thick spireme stage 
(pachytenes). The chromatin filaments seem to be thicker in the cells of 
the pig than those figured by v. Winiwarter in the rabbit. In the deepest 
layers of the cortical zone are seen primitive follicles with their enclosed 
oocyte, the chromatin filaments of which are arranged in the double-thread 
form (diplotenes). The oogonia of future follicle cells are not so numerous 
in the medullary portion as they were in the 12 cm. embryo. On the 
surface of the ovary are seen some invaginations of the capsular epithelium. 
The cells of this layer are placed with their axes at right angles to the 
underlying cells. In this embryo the nuclei in the synaptic stage are not 
numerous ; the thick spireme stage nuclei are more numerous. (Fig. 2.) 
Embryo 16 cm . — The germ cell nests are only in the superficial layers 
of the cortical zone. The capsular epithelium is isolated in places by the 
stroma cells, which reach the surface by means of a radiating distribution. 
In the medullary zone the stroma cells are loosely arranged, and there are 
large blood lacunae and vessels with thick walls. Some glandular spaces 
are seen lined with cubical epithelium — the rete tubules. The primitive 
follicles in the deeper cortical layers are very numerous. The layer of 
oogonia has disappeared from the neighbourhood of the medullary zones. 
This disappearance is accounted for by the formation of the follicles in this 
region. Mitotic figures are not numerous, and are only seen in the most 
superficial layers of the ovary. The masses of cells forming a syncytium 
are well marked in the cell nests. Some of the nuclei in these masses have 
a much more definite outline than others, and it is probable that this 
syncytial mass may be used up as a pabulum for the developing oocytes. 
The blood-vessels are nearing the periphery of the ovary. The cells in 
synapsis are very numerous. The preceding stages of resting oocytes and 
their spireme cells are few in number. Thick spireme and double thread 
stage cells are seen, the latter being enclosed in follicles. In the medullary 
zone are seen groups of cells somewhat larger than oogonia. These cells are 
probably those which have been through one stage of development and 
there become reduced in size to form cells similar to the original oogonia, 
but with a layer and more spherical nucleus. These correspond to those 
described by Miss Lane-Claypon as “ interstitial cells.” The nuclei of the 
follicle cells are oval in shape, and have their axes at right angles to the 
cell membrane of the oocyte. In some the axes of the cells are parallel to 
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