90 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



part of the embryos examined have either the intercellular vacuolation 

 or no trace of any cavity whatever. 



What is the morphological and what the physiological significance of 

 the phenomena with which we have been dealing 1 ? Are these cavities 

 all simply different phases of one and the same thing, — an ephemeral 

 recurrent cleavage cavity, — or do they differ among themselves, and are 

 they to be considered as different from the cleavage cavity 1 Do they 

 bear any relation to the conditions under which the egg develops 1 ? My 

 work has left no doubt in my own mind that they all belong to the same 

 category, — modifications of the cleavage cavity, — and that they are 

 perhaps intimately connected with the conditions in which the embryo 

 develops. I shall refer to these points again after reviewing the litera- 

 ture of the subject. 



The question might well be raised as to whether the term cleavage 

 cavity should be used to designate the phenomena which we have dis- 

 cussed. If we are to apply this term to that continuous, persistent 

 cavity into which the invagination resulting in gastrulation takes place, 

 and that only, then we assign to the term an unduly limited morpho- 

 logical significance, suggested by the Gastrsea Theory of Haeckel. Then 

 this ephemeral cavity in Limax is not a cleavage cavity, and we must 

 coin some new term to distinguish it, such, for example, as excretory 

 cavity. If, on the other hand, we recognize the physiological importance 

 of this and other cleavage cavities, while not denying their morphological 

 significance, and bear in mind also the constant intercellular nature of the 

 phenomena in question, it is in my opinion pei-fectly legitimate to desig- 

 nate by the term cleavage cavity any and all of the protean forms which 

 the intercellular space assumes in Limax. The matter of terms is, how- 

 ever, a minor point, the unity of the phenomena is the important one. 



There remains one more question of interest. Is there any relation 

 between the stages of cleavage and the recurrence of the cavity? 

 "Warneck, in 1850, stated that the cavity reaches its greatest develop- 

 ment contemporarily with the "Maximum der Entwickelung der Kerne." 

 My own observations do not show that this is always the case. In 

 Figure E, the two cells enclosing the large cavity contain, not nuclei with 

 distinct membranes, but spindles. There is a mechanical cause for the 

 elimination of the contents of the cavity at the period when the cells 

 assume a rounded condition at the close of cleavage. The facets of 

 contact are then much reduced, and the resistance at the periphery of 

 the egg to the expulsion of the fluid is more readily overcome. It may 

 also be that the periods of great activity in the cells at the time of 



