98 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Opisthobrakchiata. 



There seems to be an entire absence of references to the presence of a 

 cleavage cavity in the development of this group. 



Heymons ('93) found in Umbrella no trace whatever of a cavity at 

 any period up to the formation of the larval stage. 



Pteropoda. 



Fol ('75) says of the two-cell stage of the Pteropods, " Mais il ne se 

 produit pas ici, comme chez le Lymnee et la Limace, ou comme chez les 

 Geryonides, des vacuoles entre les cellules." Nor is a cleavage cavity 

 described by him for the later stages. Knipowitsch ('91) meutions a 

 " spaltformige und nicht immer deutlich wahrnehmbare Furchungs- 

 hohle," into which the mesodermal cells migrate, as occurring at the 

 end of cleavage in Clione. 



PULMONATA. 



Warneck ('50, pp. 131-135, 166-170) discusses the recurrent cleavage 

 cavity in Limax and Lymnseus. He describes its appearance soon after 

 the two cells begin to flatten against each other ; also its growth and 

 subsequent disappearance when the second cleavage plane appears. A 

 similar phenomenon occurs at each succeeding phase of cleavage till the 

 blastula stage is reached. He expresses the opinion that this " heller 

 Eaum," as he calls it, is a receptacle 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 excretoiy 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 Lymnseus, 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 followed by him, and no 

 further mention of the cavity occurs in his work. 



Fol ('80, pp. 115 and 116) says : " Pendant le travail du fractionne- 

 ment, les spherules prennent un aspect fonce et une forme arrondie. 

 Les noyaux ne sont plus visibles et la cavite de fractionnement se perd 

 dans robscurcissement de Foeuf. Pendant les temps de repos les noyaux 

 reparaissent, les spherules s'affaissent les unes sur les autres, la cavite 

 de segmentation est de nouveau visible. Dans ces periodes de repos, la 



