90 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
part of the embryos examined have either the intercellular vacuolation 
or no trace of any cavity whatever. 
What is the morphological and what the physiological significance of 
the phenomena with which we have been dealing? Are these cavities 
all simply different phases of one and the same thing, — an ephemeral 
recurrent cleavage cavity, — or do they differ among themselves, and are 
they to be considered as different from the cleavage cavity? Do they 
bear any relation to the conditions under which the ege develops? My 
work has left no doubt in my own mind that they all belong to the same 
category, — modifications of the cleavage cavity, — and that they are 
perhaps intimately connected with the conditions in which the embryo 
develops. I shall refer to these points again after reviewing the litera- 
ture of the subject. 
The question might well be raised as to whether the term cleavage 
cavity should be used to designate the phenomena which we have dis- 
cussed. If we are to apply this term to that continuous, persistent 
cavity into which the invagination resulting in gastrulation takes place, 
and that only, then we assign to the term an unduly limited morpho- 
logical significance, suggested by the Gastroon Theory of Haeckel. Then 
this ephemeral cavity in Limax is not a cleavage cavity, and we must 
coin some new term to distinguish it, such, for example, as excretory 
cavity. If, on the other hand, we recognize the physiological importance 
of this and other cleavage cavities, while not denying their morphological 
significance, and bear in mind also the constant intercellular nature of the 
phenomena in question, it is in my opinion perfectly legitimate to desig- 
nate by the term cleavage cavity any and all of the protean forms which 
the intercellular space assumes in Limax. The matter of terms is, how- 
ever, a minor point, the unity of the phenomena is the important one, 
There remains one more question of interest. Is there any relation 
between the stages of clenvage and the recurrence of tho cavity! 
Warneck, in 1850, stated that the cavity reaches its greatest develop- 
ment contemporarily with tho “Maximum der Entwickelung der Kerne.” 
My own observations do not show that this is always the case. In 
Figure E, the two cells enclosing tho large cavity contain, not nuclei with 
distinct membranes, but spindles. There is a mechanical cause for the 
elimination of the contents of the cavity at the period when the cells 
assume a rounded condition at the close of cleavage. The facets of 
contact are then much reduced, and the resistance at tho periphery of 
the oge to the expulsion of the fluid is more readily overcome. It may 
also be that the periods of great activity in the cells at the time of 
