494 CHARLES H. SWIFT 



oddly enough, its importance has been overlooked by all workers 

 except Rubaschkin ('08) and von Berenberg-Gossler ('12). It is 

 present in all stages, and, from the origin of the germ-cell to its 

 entrance into the indifferent gonad, does not undergo any change. 

 It appears as a condensed or flattened sphere of cytoplasm rest- 

 ing usually on the nuclear membrane, on that side of the nucleus, 

 which is the farthest removed from the cell membrane (figs. 2, 

 5 and 9). It may rest on the nuclear membrane directly, in which 

 case, it has a concavity towards the nucleus, or it may be separated 

 from the nucleus by some distance. It has an average diameter 

 of 3 to 4 micra, but in some cases where it is flattened against 

 the nucleus, its long axis may measure 6 micra. At times it is 

 very discrete and seems to be surrounded by a definite mem- 

 brane. The cytoplasm immediately around it appears less dense 

 than the rest, so that the attraction-sphere seems to be situated 

 in a vacuole. In all stages and with all stains the sphere is 

 prominent and unless obscured by yolk spheres is easily seen. 



The centrosomes are best seen after trichloracetic acid fixa- 

 tion and iron-hematoxylin staining, when they appear as black 

 dots. With this fixation cytoplasmic radiations may be ob- 

 served radiating out from the attraction sphere. 



Attraction-spheres are, of course, not confined to the primor- 

 dial germ-cells, but in the other cells they are not nearly so large, 

 distinct or constant as in the germ-cells. Because of its size, 

 discreteness and constancy, it serves as the best criterion for 

 identification of germ-cells in the chick. 



Yolk. Yolk material is a very characteristic constituent of 

 the primordial germ-cells in the chick. Its abundance in the 

 young stages has been recorded by Nussbaum ('01), Rubaschkin 

 ('08) and von Berenberg-Gossler ('12). The veiy young germ- 

 cells, just after their origin, are simply loaded with the yolk which 

 has the form of spheres; the cytoplasm is so crowded that prac- 

 tically nothing but yolk can be seen (figs. 12 and 14). Most 

 abundant in the younger germ-cells, it gradually decreases in 

 amount as the embryo increases in age. In embryos having 

 about 20 somites and thereafter, there is a great variation in the 

 quantity of yolk in the germ-cells. In some embryos the germ- 



