OOGENESIS IN THE WHITE MOUSE 267 



around under this as the developing tunica albuginea, partially 

 separating these masses from the epithelium. These groups of 

 cells retain a partial connection with the epithelium until about 

 birth when the tunica is practically complete, although not very 

 thick. At this time, the tunica is made up of one or two layers 

 of spindle-shaped 6r fusiform cells. Even in the ovary of the 

 adult mouse the tunica is not a very dense structure, consisting 

 of a few layers (3 to 6) of flattened or fusiform cells. It seems 

 to be continuous with the stroma in places where the latter is 

 radially arranged, and hence appears thicker in some regions 

 than in others. As will be seen later, the tunica has a part to 

 play in the development of the definitive ova. 



The cell-masses are made up of two kinds of cells — those with 

 large round nuclei and a relatively small amount of cytoplasm, 

 the 'primitive germ cells or oocytes,'' and smaller cells with smaller 

 round or oval nuclei, the 'indifferent cells,' which are to be re- 

 garded as the future follicle cells. In embryos or foetuses of 

 23 mm. length (ca. nineteen days post coitum) a few egg-cells in 

 the central part of the ovary possess follicles made up of a single 

 layer of these cells. As they surround an oocyte at first, they 

 are somewhat flattened, perhaps as a result of an increase in size 

 on the part of the germ cell : the follicle cells may be stretched 

 out a little before dividing to keep pace with the oocyte in 

 its growth. This follicle-formation proceeds toward the pe- 

 riphery, and is quite rapid from this time up to and after birth 

 (fig. 10). By three days after birth all the germ cells in the 

 ovary proper are surrounded by follicles, ^^hich, in the central 

 part may be two-layered, of cuboidal cells, and peripherally 

 just under the epithelium are made up of a single layer of flat- 

 tened cells. This condition persists for a few days, the centrally 

 located follicles gradually growing in size (fig. 11). In young 

 mice of 14 days, the central follicles have from three to five 



1 In this paper I use the terms 'primitive oocytes or germ cells' to refer to the 

 cells formed from the germinal epithelium in the first or embryonic proliferation; 

 'definitive oocytes or germ cells' will be applied to the egg-cells arising from the 

 germinal epithelium in the proliferation of cells between birth and sexual maturity 

 The terms 'primordial germ cells' and 'primary gonocytes' have been used by^ 

 authors in another sense and would not be appropriate here. 



