SOME PROBLEMS OF COELENTERATE ONTOGENY 519 
an early morula-like embryo. In this is shown an oval embryo 
without definitive ectoderm, or sign of entoderm. This condi- 
tion persists for some time, the only changes distinguishable 
being that of cell proliferation, or perhaps more precisely, nuclear 
proliferation; for in most cases it is not possible to distinguish 
the presence of cell boundaries of any definitive sort. But a 
most remarkable thing becomes apparent under careful staining, 
—namely, the fact that the internal mass shows a differential 
staining reaction, represented by the shaded interior. This I 
take to be indicative of an important physiological change, 
namely, an incipient entodermal differentiation directly related 
to the primary purpose of entoderm development, that of diges- 
tion. While the results do not as yet warrant a dogmatic pro- 
nouncement on this matter, they do tend to confirm a view I 
have already proposed (cf. Science, March 25, 1910). It has gen- 
erally been assumed that the ectoderm is the primary germ 
layer, and morphologically this is undoubtedly true. But if 
the suggestion just made be confirmed by later experiments one 
will have to aver that, physiologically speaking, the entodermal 
function is the first to express itself. Further consideration of 
this point will be deferred to a later section. 
a. Eetoderm. The development of this tissue is a graduated 
process. With the establishment of a surface layer of cells of 
more or less similar character one is not warranted in designating 
it as a definitive ectoderm. For, as Rittenhouse has ‘pointed 
out (07, p: 453): 
Kven those cells which are atthe surface at the completion of segmenta- 
tion cannot be regarded as primitive ectoderm, for in the breaking down 
of the cell boundaries, the formation of the syncytium, and the recast- 
ing of the cells, it is quite impossible to say what changes of proto- 
plasm may take place. 
Furthermore, it must not be overlooked that, with a primary 
layer of cells established, there are yet other ectodermal elements 
to be taken into account, such as interstitial cells, cilia, nettling 
cells, ete. Only with the formation of the supporting lamella 
ean it be claimed that the definitive ectoderm is really established. 
