88 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
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? 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 längere 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 tho 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 
lentieular 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 
