THE FORMATION OF GERM-CELLS 



195 



alo7ie are capable of giving rise to the primary germs-cells . M ore- 

 over, if we remember that in the case of the Hydroid polypes 

 the period of the separation of the primary germ-cells can be 

 relegated to earlier or later stages, it will be clear that not only 

 the cells at the terminations of the germ-track in which this 

 separation actually occurs in indi- 

 vidual cases, but also the entire 

 preceding series of cells, possess 

 qualities which are absent in the 

 other cells of the organism, and 

 which, sooner or later, render the 

 cells of the germ-track capable of 

 giving rise to primary germ-cells. 



The cells of the germ-track do not 

 themselves correspond to primary 

 germ-cells, the character of which 

 latter is not as yet apparent ; they 

 are cells of a mixed character, — that 

 is to say, they contain diilerent 

 primary constituents, which are 

 gradually separated off until event- 

 ually only two of them remain, 

 and these then separate from one 

 another by means of a final cell- 

 division. 



The embryogeny of a parasitic 

 worm (A'/iaddilis nigrove)iosa) from 

 the frog's lung may serve to illus- 

 trate this point. In fig. 15, A to D 

 represent the first four stages in 

 segmentation up to the differentia- 

 tion of the primary mesoderm-cell 

 {mes) . This and the following stages 

 are represented diagrammatically 

 in fig. 16. which shows the genea- 

 logical tree of the cells and the 

 germ-track. The ovum {Eiz) must of course be considered as 

 containing the whole of the primary constituents of the organism 

 before its first division into a primary ectoderm (t/rEkt) and a 

 primary endoderm cell {itrEnt). The latter retains all the 

 primary constituents of the mesoderm and primarv germ-cells. 



invU>. 



Fig. 1'^. — Stages in the segmen- 

 tation of the ovum and forma- 

 tion of the germinal layers in 

 Rkabditis nigrovenosa. — (After 

 Gotte.) ect. Ectoderm; ent, En- 

 doderm; mes, Mesoderm. 



