SOME PROBLEMS OF COELENTERATE ONTOGENY 509 
in the cases of Eudendrium and Pennaria, but is no less true in 
the present instances. 
4. Organization of the embryo. This has usually been assumed 
to consist fundamentally in the formation of the tissues, ectoderm 
and entoderm. In part this assumption is correct, but only in 
part. For example, the morula may remain for some time entirely 
devoid of these tissues in any definitive sense, and even in the . 
later larval stage the entoderm may not arise till.a late period. 
As has long been known, among the first evidences of organiza- 
tion is that associated with the formation of the ectoderm. 
Indeed, this is only what might naturally be expected as one 
comes to consider the primary function of such a tissue, or its 
analogue, throughout the animal kingdom. The embryo, no 
less than the adult organism, requires superficial protection against 
external conditions. And from protozoon to mammal provis- 
ion is made to this end by ectosare and epidermis, and in thé 
embryo by the ectoderm, which may be regarded as the primary 
tissue of the embryo. 
5. Entoderm formation. But up to this time there is no def- 
inite differentiation of entoderm. It is true, that one will find 
what has long been designated as entoderm, namely, an interior 
mass of embryonic matter more or less cellular, but without 
differentiation of any sort. By some students of hydroid devel- 
opment this condition has been described as the ‘end of entoderm 
formation’ (Ende der Entodermbildung). In reality one may 
better designate it as the beginning of entoderm formation, 
though even this might be open to question. What we have at 
this time is simply an interior embryonic mass, often a syncy- 
tium, within the enclosing ectoderm, if this be yet differentiated ; 
and of this mass but a very small proportion ever participates 
directly in entoderm formation. For the sake of clearness it 
seems desirable formally to recognize this condition by giving to 
it such name as may express the fact, and at present no better 
term seems available than ‘pro-entoderm.’ This only implies 
the existence at this time of material, a primordium, out of which 
in varying ways will be developed the definitive entoderm of the 
larva. 
JOURNAL OF MORPHOLOGY, VOL. 22, No. 3 
