OOGENESIS IN THE WHITE MOUSE 



269 



that the latter is completely surrounded by a layer of flattened 

 cells, a primary follicle, while still in the germinal epithelium 

 (figs. 2, 3, 38). As growth proceeds, the other cells of the 

 germinal epithelium extend up over this oocyte in its primary 

 follicle which is in this manner 'left behind' in the tunica albuginea 

 under the epithelium. The cells of the epithelium then unite 

 over the egg-cell in its follicle, closing over it (cf. figs. 1 to 6, 

 which are designed to show this) . 



These primary follicles remain in the tunica albuginea at 

 first, but as development goes on, they come to lie in the stroma 





Text-fig. 1 to 6 These arc to show the development of the follicle of a definitive 

 germ cell and the way in which the oocyte migrates from the germinal epithelium 

 into and through the tunica albuginea. In the figures, the free surface of the 

 epithelium is up and the tunica and ovary down. Figure 3 is from an ovary of a 

 mouse three days old, while the others are from the same ovarj' of a mouse twenty 

 days after birth. X 625. 



beneath it. The cells of the tunica, not a very dense layer of 

 tissue, are apparently active in this migration of the follicles. 

 They separate underneath (central to) and close up over (pe- 

 ripheral to) the follicles, and by a continuation of this process, 

 these gradually pass through the tunica and reach the stroma 

 beneath (figs. 3 to 6). New egg-cells in folhcles are being con- 

 tinually added outside these, the later formed being, of course, 

 younger than those more deeply located, which, in turn, are 

 more peripherally situated than the cells originating during 

 embryonic life, the primitive germ cells. These latter, as men- 

 tioned above, make up the bulk of the ovary for the first twenty 



