OOGENESIS IN THE WHITE MOUSE 291 



In the mouse, the mitochondria in the developing definitive 

 oocytes are almost entirely of the granular type. The cells of 

 the germinal epithelium ha^'e a small amount of cytoplasm in 

 which are a few granular mitochondria (fig. 47, upper cell). In 

 the course of the growth of one of these cells as a primary oocyte, 

 the number of mitochondria increases, keeping pace with the 

 development of the cell (fig. 47, lower cell.) The mitochondria 

 are of the granular type until the cell is quite large, in a follicle 

 of cuboidal cells, single-layered or even stratified, when rods 

 and threads begin to appear (figs. 50, 51). Thus it is evident 

 that the granular type of mitochondria prevails in the earlier 

 stages of the developing definitive oocytes. 



But in the mouse the mitochondria of the cells of the germinal 

 epithelium are also of the granular form. And further, in the 

 follicle cells surrounding the oocytes, the mitochondria, dis- 

 tributed chiefly in the part of the cells toward the oocyte, are 

 granular, rod-like, or thread-like. In the cells of some follicles 

 granules appear, and in the cells of others rods or threads are 

 found; in the same follicle, some cells may have granules and 

 others rods or threads, and it is not at all rare to find all kinds of 

 mitochondria in the same cell (fig. 52). It is seen, then, that, 

 in the mouse, while the granular type of mitochondria is pre- 

 dominent, perhaps, in the developing definitive oocytes, this 

 is not a distinctive feature, for epithelial cells and follicle cells 

 as well have a similar mitochondrial content. 



This might be expected from the work of Lewis and Lewis 

 ('15) on mitochondria in tissue cultures. They find that the 

 mitochondria can be observed in the living cell, unstained, and 

 that these mitochondria are not constant in form, but change 

 their shape repeatedly. Rods or threads may be seen to break 

 up into granules, and granules fuse to form larger granules. 

 Accordingly, one would not expect to find the mitochondria of 

 any one shape constant in or peculiar to any particular kind of 

 cell. 



Schaxel ('11) states that the shape of the mitochondria varies 

 according to the method of fixation and staining: — that with the 

 Benda technique, rods or threads predominate, while after 



