OOGENESIS IN THE WHITE MOUSE 277 



The development of these germ cells, the definitive oocytes, 

 is marked chiefly, perhaps, by the utter absence of the compli- 

 cated chromosomal history which is usually associated with this 

 process. In the description to follow, it seems convenient to 

 speak of three stages in the development, 'a,,' 'b,' and 'c;' these 

 are, of course, more or less arbitrary, and are not sharply dis- 

 tinguished, one passing almost insensibly into the next. 



Apparently any cell in the germinal epithelium is a potential 

 egg-cell. From the cells of this epithelium develop oocytes, 

 follicle cells, and residual germinal epithelial cells; and at first 

 there is no way of distinguishing the different kinds of cells or 

 their potentialities. At first neither the mitochondrial content 

 nor the nuclear structure is distinctive or characteristic of any 

 one of these possible lines of development. It is not until one 

 of these cells, in the course of its differentiation, begins to grow 

 in size that a germ cell can be distinguished from other cells 

 in the epithelium. It would be difficult to determine just what 

 the factors are which determine the line of differentiation an}'* 

 cell of the epithelium will take. 



To digress for a moment: Jenkinson ('13) states that the 

 "oocytes of the outermost layer often lie practically in the 

 epithelium," but he thinks that they have developed from the 

 'primordial germ cells' which have migrated in at the hilum of the 

 ovary. There can be no doubt whatever that in the mouse the 

 oocytes start their development in the epithelium. Figures 35, 

 36 and 38 have been drawn with the adjacent -cells of the epithe- 

 hum to bring out this point. In figure 35, in which, through 

 shrinkage, the epithelium has been slightly torn away from the 

 ovary, it is shown conclusively, I think, that the egg-cells are 

 actually in and a part of the epithelium and not merely crowded 

 against the basal side of it. 



Stage 'a.' The cells of the germinal epithelium have, of course, 

 been di\dding during the growth of the ovary, keeping pace with 

 its increase in size. This might be termed the "multiplication 

 period' (figs. 25 to 27). Beginning about three days after birth, 

 certain of these epithelial cells commence to grow in size, and 

 from then on they may be considered oocytes (primary) since 



JOURNAL OF MORPHOLOGY, VOL. 30, NO. 1 



