94 GERM-CELL CYCLE IN ANIMALS 



the germ-cells is that the cells invariably arise in 

 the ectoderm (kz" rf ), 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 Hydr actinia. 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 



