88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



absence of the secondary intercellular cavities in the older egg. It 

 seems probable that, as the central cavity grows in volume and the 

 facets of contact diminish in size, the central cavity extends to these 

 secondary cavities and fuses with them, and that thus all portions of the 

 surfaces of the cell, except its exterior one, may contribute secretions to 

 the central cavity. The immediate proximity of several of these sec- 

 ondary cavities to the central cavity in Figures 24 and 25 (Plate III.) 

 suggests that the former may even actually move toward the central 

 cavity. The direction of the motion is merely a question of the direc- 

 tion of least resistance ; it is difficult to explain the development of such 

 a large cavity as that of Figure 34 (Plate V.) and the subsequent forci- 

 ble expulsion of its contents, and the immediate restoration of the egg 

 to a solid spherical mass, without admitting the existence of a consider- 

 able force, tending to preserve intact the contour of the egg, and resist- 

 ing the increasing tension brought about by the enlargement of the 

 central cavity. If the contents of these smaller cavities are eliminated 

 to the exterior, why should not those of the large cavity, whose tension 

 must be proportionally greater, be eliminated at the same time t There 

 is a point, however, beyond which the increase in the size of the cen- 

 tral cavity cannot go. The outer wall yields to the pressure, and the 

 imprisoned fluid escapes. I have found no trace of an egg membrane, 

 such as Gegenbaur ('52) has described for the egg of Limax agrestis : 

 " Es besitzt eine Zellmembran, die besonders durch langere Einwirkung 

 von Wasser deutlich erkennbar wird." There is no evidence that there 

 is anything more than the ordinary clear stratum of protoplasm at 

 the exterior of the egg. In this my observations are in accord with 

 those of Dr. Mark upon Limax campestris ('81). None of these secon- 

 dary cavities or lenticular spaces were observed in the eggs which have 

 the maximum central cavity, neither have they been seen at the time of 

 the expulsion of the contents of the cavity, even when that takes place 

 gradually. They are associated with the growth rather than with the 

 disappearance of the central cavity. In view of these facts, it seems to 

 me that we are justified in concluding that, in some cases at least, the 

 lenticular and the secondary intercellular spaces contribute to the 

 increase of a central cavity. 



As has been stated already, these secondary intercellular spaces often 

 form at the animal pole of the egg, while not a trace of them can be 

 found at the vegetative pole. They may present the appearance of an 

 anastomosing network of irregular vessel-like structures between the 

 cells of that pole of the egg, as in Plate VI. Fig. 39. It hardly seems 



