SOME PROBLEMS OF COELENTERATE ONTOGENY 521 
becoming more capacious. As the entoderm cells become defi- 
nitely organized and adjusted in contact with the supporting lamella 
the entoderm may be said to be established as a tissue. But this 
does not become complete until a large proportion of the pro- 
entoderm mass has been reduced and appropriated by the embryo. 
There yet remains masses of cells in the cavity along with yolk 
fragments and other debris variously distributed. 
Earlier accounts of the differentiation of the entoderm differ 
in several particulars from that here given. In the first place, 
it has been generally assumed that the entoderm is early estab- 
lished, an error which I have taken occasion in several accounts 
to correct. In the next place, the exact mode of its differentiation 
has not been very critically studied, nor the fate of those parts 
of the pro-entoderm not directly concerned in entoderm forma- 
tion. For example, Korschelt and Heider, following the older 
accounts, have asserted that following the establishment of the 
entoderm the remaining cell-mass undergoes fatty degeneration, 
serving in part as food matter, with a residual mass of debris, 
the fate of which is not formally stated. I have not found in 
my preparations any evidence of such fatty degeneration, though, 
as stated above, I have found direct evidence of the operation 
of digestive ferments. According to Wilson (’83) something 
akin to amoeboid engulfment of these cells and their intracellular 
digestion is tentatively suggested: 
6 
These appearances suggest, though they do not prove, that the yolk 
granules and spheroids pass bodily into the cells. I have never seen 
them in the act of passing into the cells, but the technical difficulties 
are great, and the other considerations seem sufficient weight to warrant 
the provisional acceptance of the view advanced. 
That something of such amoeboid engulfment may occur is 
not altogether improbable; though I have found slight evidence 
of it. We know that in the growth of the oocyte in certain spe- 
cies just such a process does take place, and its occurrence in the 
slightly later history of the embryo would be what might naturally 
be expected. Indeed, I have, in an earlier part of this paper, 
suggested such a process in the behavior of the cells of the pro- 
