194 



THE GERM-PLASM 



primary germ-cells frequently take very different courses : they 

 are in some cases very short, and in others longer — sometimes 

 so long that they pass through very different embryonic cells ; 

 in some instances they branch off from the primary endoderm- 

 cells, and in others from those of the mesoderm, and they may 

 even arise from later generations of the mesoderm, ectoderm, or 

 endoderm-cells. 



Fig. h.— Three Stages in the development of the summer eggs of Moina.— 

 (After Grobben.) A, Stage of segmentation viewed from the vegetative pole, 

 in which thirty-two cells are present; B, Blastula stage, from the vegetative pole; 

 C, Gastrula stage, in longitudinal section; g, the primitive germ-cells. (From 

 Korshelt and Heider's ' Lehrbuch der vergleichenden Entwickelungsgeschichte.') 



The same germ-track is always strictly followed in each of 

 these cases respectively, no deviation ever taking place : thus 

 the primary germ-cells never arise from endoderm-cells in a 

 group in which the normal germ-track lies in the ectoderm, 

 and vice versa. We consequently cannot help arriving at the 

 conclusion that the cells in the germ-track must have some 

 advantage over the rest of the cell-tracks in ontogeny, for they 



