96 bulletin: museum of comparative zoology. 



appears entirely. But his figures and descriptions show that this clear 

 area merely occupies a different position with reference to the first 

 micromeres, 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, wdiich is in his 

 view the true 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 ('83) finds a definite cavity appearing in Dentalium as 

 early as the eiglit-cell stage. This gradually increases in size, forming 

 quite a large cavity at the time of gastrulation. 



PROSOliRANCIIIATA. 



I. 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. 



McMurrich ('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 



