1910 - 11 . J Development of Germ Cells in Mammalian Ovary. 169 
of the ovary, some having large clear nuclei with nucleoli. The follicle 
cells near the deeper portions are spindle-shaped in outline, those nearer the 
surface being columnar or cubical. The deeper follicle cells proliferate 
rapidly and form several rows, and the follicles have the appearance of fully 
developed graafian follicles. These rows of cells become separated, an 
inner row adhering to the cell membrane of the oocyte ; vacuoles form 
between the cells, and they appear irregular in their distribution. In the 
younger follicles the nuclei of the oocyte contain long double, filaments, but 
in the more advanced stages the filaments are short and in the form of 
rings, figures of eight, and even tetrads. In some cells an organ of Balbiani 
can be made out. 
E. Human Foetal Ovary — fourth month. 
The ovary at this stage is a long narrow body, showing on transverse 
section an oval or triangular outline. One pole is broader than the other, 
the cortex and medulla are differentiated, the medulla consisting of loose 
stroma and blood-vessels. In the deeper cortical layers are numerous 
follicles, some solitary, others in groups. The cortical zone is broad, and 
near its periphery are numerous cell nests, but these groups are not so 
easily differentiated as in other mammalian types, owing to the stroma 
being in very fine strands. At the periphery is a dense layer of cells, not 
divided into groups. The axes of the cells of the capsular epithelium are 
perpendicular to the underlying cells. The nuclei are oval or columnar 
in shape and are granular. Early invaginations of the epithelium are seen. 
Although the stroma cells have not reached the capsular epithelium, it can 
be differentiated easily from the underlying cells by a band of protoplasm, 
somewhat hyaline in appearance. There are numerous oocytes of the first 
stage in the peripheral layer, but the main number of cells are in the stage 
of synapsis, and also in the thick spireme stage. A few are seen undergoing 
transformation from the reticular or first-stage oocyte to the fine spireme. 
The chromatin filaments appear thick in the synaptic stages, and they have 
a beaded appearance. In the early synaptic stages the filaments appear fine 
and double, in the later they are thick and single. In the deeper layers of the 
cortical zone are long twisted double filament nuclei, with nodes of chro- 
matin. The nucleolus is present during the whole development of the 
oocyte, but is faint in outline in the synaptic stage, where it is often lost 
to all appearance in the skein of filaments. It is well defined in the double- 
thread stage. Sometimes more than one nucleolus is seen, but a principal 
nucleolus is always to be made out in an eccentric position. The chromatin 
masses are irregular in size, but I have not seen the same outlines as those 
