102 ■ BULLETIN : MUSEUM OF COMPAKATIVE ZOOLOGY, 



structures. No sections of this stage are figured, and the relation of these 

 canals to cell boundaries is not determined. The fact that the presence 

 of the canals obscures the boundaries between the cells, and that these 

 canals are in continuity, suggests the possibility that they may be inter- 

 cellular and therefore merely an exaggerated form of the anastomosing 

 intercellular spaces so common in Limax. Yejdovsky does not suggest 

 their relationship to the cleavage cavity, neither indeed does he regard 

 a space found in the two-cell stage as having anything to do with that 

 cavity. His grounds for this view, and his explanation of the phenome- 

 non, are as follows (p. 105): "Die Hohle zwischen beiden Furchungs- 

 kugeln ist als Ueberrest der Vorgange zu betrachten, die sich bei der 

 Bildung der Zellmembranen beider Furchungskugeln abgespielt haben. 

 Diese Hohle zwischen den ei'sten 2 Furchungskugeln ist bereits oft beo- 

 tachtet und als eine primare Furchungshohle (!) gedeutet worden. Es 

 ist liberfllissig eine solche Auffassung zurtickzuweisen, einmal, dass es un- 

 moglicb ist, dass eine Furchungshohle bereits zwischen zwei ganz gleich 

 gestalteten Furchungskugeln zum Vorschein kommen konnte, ein ander- 

 esmal, dass derartige Hohle ofters auch wahrend des spatereu Furch- 

 ungsprocesses zwischen je zwei Kugeln zum Vorschein kommt (vergl. 

 Taf. IX. Fig. 11, 14). Gewiss ist diese Erscheinung von den Verhalt- 

 nissen der Zell- und Kern-platte abhangig." 



In the absence of the evidence upon which these opinions rest, it 

 seems superfluous to discuss them. His suggestion that the formation 

 of the cavity of the two-cell stage is dependent upon the phenomena 

 of the division resulting in that stage is certainly not sustained by the 

 facts. If his opinion were the correct one, we should find a similar 

 cavity in the two-cell stages of all forms, fresh-water and mai'iue alike. 



The preceding review of the literature shows that ^Yarneck ('50), 

 Eabl (79), Fol ('80), and Brooks ('80) have all noted the recurrent 

 character of a cavity in the early stages of cleavage in the Pulmonates, 

 but the three later writers have added little to the admirable observa- 

 tions of the first named investigator. 



A glance at the summary of the literature on Prosobranch develop- 

 ment shows an entire absence of any refei'ence to a recurrent segmen- 

 tation cavity in the marine forms, unless an exception be made with 

 regard to McMurrich's observations on the cavity in Fulgur. When a 

 cleavage cavity does occur, it appears at a very late stage in the seg- 

 mentation, is comparatively small, and is never recurrent. The cleavage 

 of the fresh-water Prosobranchs has not been fully studied except in 



