520 CHARLES W. HARGITT 
b. Entoderm. As in the case of the ectoderm, what has 
been said as to entoderm formation in Hydractinia will apply 
for the most part to Clava. What has been said above in ref- 
’ erence to the morula as the primordial embryo applies in this 
connection. Entoderm formation is a graduated process, and 
in its morphology a much slower process than that of the ectoderm. 
In its physiological genesis it may be said to outrun the ectoderm; 
for its functions begin almost immediately after the completion 
of cleavage. As was pointed out in an earlier section, the inter- 
nal cell-mass included within the primordial ectoderm is not in 
any sense a tissue, but rather a primordium,—pro-entoderm. 
For some time following the nuclear proliferation of this mass 
continues. But at the same time another, and extremely differ- 
ent process is under way, namely, that of cellular and nuclear 
disintegration and destruction. Out of this interior mass rela- 
tively few cells will survive to constitute the definitive entoderm 
of the polyp. What is taking place is in reality a struggle among 
these cells for nutrition, reminding one of the ingenious theory 
of Roux (81), ‘Der Kampf der Theile im Organismus,’ though, 
so far as I am aware,.this author never applied his theory in this 
direction. It is not until after the planula has become free that 
a definitive entoderm is finally established; indeed, this does not 
become established until near the metamorphosis of the planula 
into a polyp, though one may trace stages in the process much 
earlier.” What really happens is that the same sort of vicarious 
process of nutrition occurs as that by which, inmany hydroids, 
the oocyte grows; that is, the devouring of sister cells or primor- 
dial ova; in the later stage occurs the similar process of digesting 
the pro-entoderm cells and making their substance available as 
nutrition to the embryo. As is well known, these pro-entoderm 
cells are richly laden with yolk granules, as were also originally 
the cells of the ectoderm. But long after the ectoderm has ex- 
hausted this primitive supply the entoderm is reducing its sur- 
plus cell mass for similar ends. 
With the gradual advance of this process the coelenteron of 
the larva grows larger, appearing in sections as an axial slit of 
irregular outline, and later assuming a more regular aspect and 
