ORIGIN OF PRIMORDIAL GERM CELLS 495 



cells may all have no yolk, while in others, some will have yolk 

 while the remainder will have none at all. 



In the embiyos of moderate age — 20 to 26 somites — there is 

 evidence that the yolk is undergoing digestion, as is shown by 

 the fact that in the same primordial germ-cell fixed with osmic 

 acid some of the yolk spheres will be intensely black while others 

 will be brown or yellowish. The yolk spheres which are not black 

 in these cells are frequently surrounded by a clear ring. 



In the embryos of about 20 to 26 somites, the yolk spheres 

 reach their greatest size (fig. 7). In one case one had a diameter 

 of 8 micra. In the younger germ-cells the yolk spheres are smaller 

 having an average diameter of 2 to 3 micra. In embryos with 

 33 somites to four and one-half days the yolk content varies a 

 great deal; usually there is only a small quantity present, which 

 is confined to 5 to 10 small spheres, which usually show signs of 

 digestion. 



In the very young primordial germ-cells the yolk is scattered 

 evenly throughout the germ-cell, usually entirely surrounding the 

 nucleus, which may have a central rather than an eccentric po- 

 sition. As the germ-cell grows older the yolk diminishes in 

 amount and retreats to one side of the nucleus, usually the cell 

 pole which is occupied by the attraction-sphere (figs. 7 and 9). 

 As the yolk decreases still more in amount it is confined to the 

 region between the attraction-sphere and the cell membrane. 

 At this stage the yolk spheres are apt to be arranged in a horse- 

 shoe shaped manner, with the concavity turned towards the 

 attraction-sphere (fig. 6). 



The yolk remains in the primordial germ-cells long after it 

 has disappeared from the tissue cells. The presence of the yolk 

 in the germ-cells is probably related to the absence of mitoses in 

 these cells. 



Mitochondria. The mitochondria in the primordial germ-cells 

 of the chick are not at all characteristic. They resemble the 

 mitochondria of the somatic cells (figs. 5, 8 and 11), and at the 

 same time retain the same appearance, whether in the primitive 

 streak stage or four and one-half day chick. They are seen usu- 

 ally to be in the form of short rods of a length varying from 1 to 



THE AMERICAN- JOURNAL OF AVATOMY, VOL. 15, NO. 4 



