98 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
OPISTHOBRANCHIATA, 
There seems to be an entire absence of references to tho presenco of a 
cleavage cavity in the development of this group. 
Heymons ('93) found in Umbrolla no trace whatever of a cavity at 
any period up to the formation of the larval stago. 
PTEROPODA. 
Fol (75) says of the two-cell stage of the Pteropods, ** Mais il ne se 
produit pas ici, comme chez le Lymnée et la Limace, ou comme chez les 
Geryonides, des vacuoles entro les cellules.” Nor is a cleavage cavity 
described by him for the later stages. Knipowitsch (91) mentions a 
* spaltfórmige und nicht immer deutlich wahrnehmbare Furchungs- 
höhle,” into which the mesodermal cells migrate, as occurring ab the 
end of cleavage in Clione. 
PULMONATA. 
Warneck (550, pp. 131-135, 166-170) discusses the recurrent cleavago 
cavity in Limax and Lymneus. He describes its appearance soon after 
the two cells begin to flatten against each other ; also its growth and 
subsequent disappearance when tho second cleavage plane appears. A 
similar phenomenon occurs at each succeeding phase of cleavage till tho 
blastula stage is reached. He expresses the opinion that this “ heller 
Raum," as he calls it, is a recoptacle for albumen, and describes the 
expulsion of its albumen-like contents into the surrounding albumen at 
the time of the disappearance of the cavity. He explains the phenome- 
non as due to the acceleration of end- and ex-osmosis, attendant upon the 
greater activity of the nutritive and excretory functions of the cells and 
the disappearance and reappearance of the nucleus during the successive 
phases of cleavage, and correlates this activity of the cells with the 
origin of the ovum from a glandular tissue. Ganin (73) mentions the 
relatively small cavity in Lymnmus, and the larger cavity in Physa. 
In Helix, von Jhering ('75) finds a central cavity in the two-cell stage. 
The later stages of cleavage were not carefully followod by him, and. no 
further mention of tho cavity occurs in his work. 
Fol (’80, pp. 115 and 116) says: € Pendant le travail du fractionne- 
ment, les sphérules prennent un aspect foncé et une forme arrondie. 
Les noyaux ne sont plus visibles et la cavité de fractionnement se perd 
dans l'obscurcissement de l'ouf. Pendant les temps de repos les noyaux 
reparaissent, les sphórules s'affaissent les unes sur les autres, la cavité 
dé segmentation est de nouveau visible, Dans ces périodes de repos, la 
