290 H.,M. KINGERY 



and ordinary cells of the germinal epithelium on the one hand, 

 and between these cells and oocytes in mature Graafian follicles 

 on the other. The evidence, then, presented by a study of a 

 series of mouse ovaries between birth and sexual maturity shows 

 rather conclusively that there is a new formation of germ cells 

 after birth, and that the definitive ova come from these cells. 

 ■ Rubaschkin ('12) from his work on the guinea pig states that 

 developing egg-cells may be distinguished from other cells in the 

 ovary by a difference in the mitochondrial content. Mitochon- 

 dria in the germ cells appear exclusively in the form of granules, 

 while in other cells, including those of the germinal epithelium 

 rods or threads are found. He thus distinguishes sharply be- 

 tween germ cells and epithelial cells, in accordance with his 

 theory of the 'Keimbahn.' The granular form is the type found 

 in the embryonic cells; as differentiation of the cells proceeds the 

 mitochondria become transformed into rods and threads. The 

 germ cells, accordingly, show their embryonic or undifferentiated 

 condition by the granular type of their mitochondria. 



Firket ('14) finds that, in the chick, the type of mitochondria 

 is not constant in the germ cells. Swift ('14) states that the 

 mitochondria in the 'primordial germ cells' of the chick are 

 usually rods, although granules are frequently present also. 

 This is directly opposed to the results of Tschaschin ('10), a 

 student of Rubaschkin, who describes the mitochondria of the 

 'primordial germ cells' of the chick as exclusively granular. 



In this connection the work of Lewis and Robertson ('16) 

 is of interest. In tissue cultures of the testicular follicles of the 

 grasshopper, Chorthippus curtipennis, the mitochondria were 

 observed in the living germ cells in the course of their develop- 

 ment. Granular in the primary spermatagonia, the mitochondria 

 become granular threads in the secondary spermatogonia and 

 assume a long thread-like appearance during mitosis. In the 

 growth period of the primary spermatocytes, the mitochondria 

 are again of the granular form. This would show that in this 

 form the shape of the, mitochondria is not constant in the germ 

 cells during their development. 



