96 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
appears entirely. But his figures and descriptions show that this clear 
area meroly oceupies a different position with reference to the first 
micromeros, not that it entirely disappears. Every stage that he figures, 
from the two- to the thirteen-cell stage, where, upon his interpretation, 
the true cleavage cavity first appears, contains a cavity. He brings 
forward no proof to show that these may not be continuous both with 
one another and with the cavity of the thirteen-cell stage, which is in his 
view the trae cleavage cavity. It seems to me, then, that his own evi- 
dence does not conclusively sustain the view that this primitive cavity 
is not a true cleavage cavity, as he himself has defined it. Indeed, we 
should expect that in Cyclas, as in other fresh-water mollusks, there 
might be an entire elimination of the cavity at intervals, though he has 
not proved it. Even if we grant that in Cyclas the primitive cavity is 
eliminated, we have still the important point to consider whether or not 
such an elimination constitutes a valid ground for removing the * heller 
Raum" from the category of cleavage cavities. A comparison of the 
phenomena in Cyclas with those presented in such a form as Limax 
would seem to indicate that we are dealing here, as there, with an 
ephemeral recurrent cleavage cavity present at the very beginning of 
segmentation. 
SCAPHOPODA. 
Kowalevsky (85) finds a definite cavity appearing in Dentalium as 
early as the eight-cell stage. This gradually increases in size, forming 
quite a large cavity at the time of gastrulation. 
PROSOBRANCHIATA, 
L Marine Forms. 
An examination of the literature of Prosobranch development shows 
an almost entire absence of references to a cleavage cavity. The few 
allusions that exist are concerned with the cavity that appears com- 
paratively late in the period of cleavage. 
Bobretsky (77) finds a cleavage cavity in Nassa mutabilis at the 
thirty-six-cell stage. Although the alternation of the rounded with 
the flattened conditions of the cells in cleavage is quite prominent in 
Nassa, no cavity is noted as occurring between the fused cleavage 
spheres. 
MeMurrich (86, p. 412) makes the following statement with regard to 
the segmentation cavity in Fulgur : “To one side of the blastoderm and 
below it a more or less distinct cavity is to be seen, containing granular 
