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 (sf) 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 (£72). 
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 
