522 CHARLES W. HARGITT 
entoderm during differentiation. But associated with the proc- 
ess there are strong evidences of the action of digestive ferments 
which are set free by these cells in which this process is first set 
up and carried forward. This likewise takes place in the case of 
the oocyte during growth, as has been shown by many recent 
observers. The suggestion of Metschnikoff long ago, that intra- 
cellular digestion forms the dominant, if not the only digestive 
process in coelenterates, is not borne out by recent investigations. 
For example, it is well known that medusae, actinians, etc., 
capture highly organized prey, such as crustacea, fish, etc., and 
digest it quite after the fashion of the higher Metazoa. The 
same thing is easily demonstrated in hydroids, in which small 
organisms, like worms, copepods, etc., form an important food 
supply. Gland cells are well known in the entoderm of Hydro- 
zoa and are evidently associated with digestive functions. Hence 
it seems more than probable that enzyme digestion is no less a 
feature in this than in other animal groups; and that it more than 
any other, is the mode involved in the reduction of this inner 
cell mass of the planula is almost certain. This in no wise 
contravenes the fact of the presence of yolk granules in the ento- 
derm cells, for they were original constituents of these cells, just 
as in the case of the primordial ectoderm cells. Whether such 
yolk granules are ever taken in entire by the larval entoderm may 
be open to doubt, at least till better sustained by direct evidence 
than at present. 
So far as I am aware, the general conception herein outlined 
as to the physiological processes involved in this phase of larval 
development has not been hitherto proposed. Of its fundamen- 
tal validity there seems no serious objection and much direct 
evidence. In brief, it involves these facts: First, that of the 
pro-entoderm mass of cells relatively few go to constitute the defin- 
itive entoderm of the planula. In the second place, the primary 
process involved in the necessary differentiation must be one of 
selection. So far as one can distinguish these pro-entoderm cells 
are alike in form and potency. The primary demand in embry- 
ogeny is growth, which involves nutritive material in some form. 
And the only source of such is that associated with these cells 
