No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 385 



facts, taken in connection with the antecedent improbability of 

 Fujita's conclusion, make it appear very probable that the &gg 

 of Siphonaria contains but the usual number of quartettes of 

 ectomeres. 



Cleavage Cavity and Vacuoles. 



The existence of a recurrent cleavage cavity seems to char- 

 acterize, in an especial manner, the cleavage of the pulmonate 

 gasteropods. It has been observed by several workers on these 

 forms (Warneck, Fol, Rabl, Brooks, Schmidt, Meisenheimer), 

 and has been described so fully in the case of Limax by Kofoid, 

 that it will only be briefly considered here. In Planorbis the 

 cleavage cavity does not attain nearly such extensive develop- 

 ment as in Limax. Whether this relation obtains between the 

 aquatic and terrestrial pulmonates in general is uncertain, though 

 in the aquatic forms, Physa, Lymnaea, and Planorbis, the cleav- 

 age cavity is not so large as in the terrestrial genera, Limax and 

 Succinea. In Planorbis, as in all the above forms, the cleavage 

 cavity first appears in the two-cell stage. This disappears at 

 the next division ; and, in the four-cell stage, two cavities appear 

 between the two pairs of cells yij5 and CD, and, in addition, there 

 develops a central cleavage cavity which assumes a quadrate 

 form and finally merges with the other two. A similar occur- 

 rence repeats itself at the eight-cell stage. With each cleavage 

 the cavity disappears, then forms again, and gradually increases 

 in size until the rounding off of the cells during the next cleav- 

 age causes a break in the wall and allows the fluid contents to 

 escape. From the twenty-four-cell stage on, the cleavage cavity 

 appears to be permanent. 



Small intercellular vacuoles occur in a late period of cleavage, 

 as in Limax, and they are found most abundantly in the ecto- 

 dermic portions of the ^gg. All of these cavities, as pointed 

 out by Kofoid, seem to be the result of excretory activity. 

 (For a discussion of the function and occurrence of the cleavage 

 cavity in different forms, see Kofoid ('95), p. 81.) 



