94 GERM-CELL CYCLE IN ANIMALS 



the germ-cells is that the cells invariably arise in 

 the ectoderm {kz^'"), then pierce through the sup- 

 porting lamella {st) into the endoderm {kz'")^ and 

 then creep along it to their maturing-place. Once 

 there, they break through again to the outer layer of 

 cells, the ectoderm {kz), and come to maturity {Ei). 

 That they make their way through the endoderm is 

 probably to be explained by the fact that they are 

 there in direct proximity to the food-stream which 

 flows through the colony {GH = gastric cavity), 

 and they are thus more richly nourished there than 

 in the ectoderm. But, although this is the case, 

 they never arise in the endoderm; in no single 

 case is the birthplace of the germ-cells to be found 

 in the endoderm, but always in the ectoderm, no 

 matter how far back it may have been shunted. 

 Even when the germ-cells migrate through the en- 

 doderm, their first recognizable appearance is in- 

 variably in the ectoderm, as, for instance, in Podo- 

 coryne and Hydractinia. The course of affairs is 

 thus exactly what it would necessarily be if our 

 supposition were correct, that only definite cell- 

 generations — in this case the ectoderm-cells — 

 contain the complete germ-plasm. If the endoderm- 

 cells also contained germ-plasm it would be hard 

 to understand why the germ-cells never arise from 

 them, since their situation offers much better con- 

 ditions for their further development than that of 

 the ectoderm-cells. It would also be hard to under- 

 stand why such a circuitous route was chosen as that 

 exhibited by the migration of the young germ-cells 



