534 CHARLES W. HARGITT 
d. The blastocoel. As another phase of the germ-layer prob- 
lem, the cleavage-cavity calls for some passing notice. Formerly 
it had large attention at the hands of embryologists, and, though 
less emphasized at present, it has not passed out of consideration. 
One can hardly consult a current paper dealing with early devel- 
opment without meeting the problem of the origin of the cleavage 
cavity and its later fate in ontogeny. It is not necessary that 
one should assume to discredit entirely any possible morpho- 
genic significance to this cavity in any group of organisms; but 
one does not need to study any considerable series of onto- 
genies to have forced upon him the conviction that its importance 
has been greatly exaggerated and correspondingly misinterpreted. 
One of the first impressions to be gathered from any considerable 
comparison of coelenterate embryology is that of the conspicuous 
absence of any definite blastocoel. It is only necessary to cite 
such figures as those given on plates I to III, illustrating these 
phases in Pennaria, Tubularia, Clava, Hydractinia to make 
this point very evident. It is true that here and there at certain 
stages of cleavage may be found irregular intercellular spaces 
which have been designated in general as segmentation cavities by 
those describing them. Spaces they undoubtedly are; but they 
are not cavities which have any permanence, either of form or 
position, but shift, or disappear under the erratic adjustments 
of the blastomeres; and one might about as well speak of the mor- 
phologic significance of the interstices in a box of oranges or bag 
of potatoes as of these promiscuous intercellular spaces. 
Another feature may also be mentioned in this connection. 
That is the rather significant fact that in many species, such as 
Hydractinia, where during very early cleavage a cavity may 
appear incidentally, it almost immediately disappears, becoming 
totally and permanently obliterated by encroaching cells. And 
even in certain species, where a more or less characteristic cavity 
arises and persists for a time, as in certain Geryonids, I am 
constrained to interpret it as having a physiological rather than 
morphological significance,—a sort of embryonic receptacle for 
the deposition of cytolymph, or other substances developed dur- 
ing cleavage, or possibly for food matters derived from the water 
