362 MONTGOMERY. [Vor. XII. 
Thus, though under the term “archicoel”’ is understood the 
cleavage cavity between the embryonic layers ectoderm and 
entoderm, it is in reality also equivalent to the space between 
any two neighboring blastomeres, as, ¢.g., the cavity between 
two ectodermal or between two entodermal cells, or even 
between an ectodermal and an entodermal cell. Further, since 
this cleavage cavity is, in pre-blastula stages at least, in a direct 
communication with the outside (as is well seen in the cleavage 
of the Ctenophora and Tricladidea), its relation to the gastro- 
coel is found to be close. And in the very frequent, and 
perhaps most primitive, method of gastrulation, according to 
which the entodermal cells delaminate from the ectoderm and 
wander into the blastocoel, we find that the cavity enclosed by 
the later entoderm, and thus comparable to a gastrocoel, is, in 
fact, a portion of the earlier blastocoel. So, nearly all intergra- 
dations may be found between the cleavage illustrated by the 
triclad Turbellaria, where the blastomeres are at first isolated 
(with consequently a large and open cleavage cavity), and the 
sterroblastic cleavage exhibited, e¢.g., by the polyclad Turbellaria 
or the Rotatoria, where the blastocoel is represented merely by 
clefts between adjacent blastomeres. 
Gastrocoel and blastocoel are nothing more than spaces 
between or enclosed by the blastomeres, which, in early stages 
at least, communicate with the outside. The gastrocoel may 
be in certain cases a space lined by entodermal cells, divided 
off from the primitive blastocoel, or it may be an extraneous 
space bounded by such cells. In the same group of animals 
both modifications may occur (e.g., in the Mollusca, the Crus- 
tacea, and Turbellaria). 
The embryonic body cavities known as blastocoel and gas- 
trocoel are, therefore, not morphologically distinct spaces, and 
only in certain cases (e.g., typical invagination gastrulation) are 
they to be separated. For the mode of formation of both is 
apparently dependent upon such factors as the mechanical pres- 
sure of the yolk, etc. (other factors might well be at work, 
which we have failed to recognize), that is, dependent upon the 
process of cleavage; and the latter process, as is well known, 
may present great differences in closely allied forms (as in the 
