KOFOID: DEVELOPMENT OF LIMAX. 85 
narrow, and the individual cells are in optical section somewhat lozenge- 
shaped. The cell aë? (Fig. 34) presents a curious bud-like process 
extending into the central eavity, and the superficial extent of the cell 
is somewhat less than that of the other members of the same quartet. 
This process suggests the mesenehyma cells which Stauffacher (93) 
figures in his Tafel XIV. Figo. 25a and 25 c, but in this projection 
there is not the least trace of any nuclear structure, and it is probably 
a mere amceboid outgrowth of no permanent significance, 
A comparison of the computed volumes of the whole egg, of its 
cavity, and of the protoplasmic portion, with the volume of another 
egg (Plato 1V. Fig. 27) of the same stage but having no cavity, brines 
out the following results. The whole egg has 429 units of volume, of 
which 188 represent that of the cavity, 241 that of the blastula wall, 
while the twenty-four-cell stage of average sizo (Fig. 27) has a volume 
of only 126 units. Those figures assume the perfect spherieity of the 
objects measured, and are therefore only approximately correct; still 
they show that the first egg, though a largo one, is within the limits 
of variation in sizo, aud that tho cavity is larger than the average egg, 
but not so voluminous as the substance of tho egg which contains it. 
It is also suggested, in view of the lareo size of the egg, that the envity 
has not, been developed to any great extent at the expense of thé volume 
of the protoplasm of the egg. There can be no question that this egg 
presents the condition of a typical “ blastula” with a typical “ cleavage 
cavity " or blastocœl. Indeed, Rabl could not have found for Haeckel 
and his Gastræa Theory a better illustration among mollusks of the 
“morula” and “blastula ” stages than these two twenty-four-cell stages 
(Plate IV. Fig. 27 and Plate V. Fig, 34); for the first contains no 
cavity whatever, and the latter has its cells arranged in a sinple layer 
about a cavity. On the other hand, if we accept the limitation set upon 
our usage of the term cleavage cavity by Stauffacher in his recent paper 
(93), we shall be compelled, in view of the fact that the envity is sooner 
or later entirely eliminated, to call this beautiful example of a cleavage 
cavity simply “ ein heller Raum.” 
It is difficult to establish any regularity or uniformity in the sequence 
of the phases of the cavity in these later stagos of cleavage. When we 
examine other eggs in the twenty-four-cell stage we meet with different 
and by no means constant conditions. The twenty-four-cell stage repre- 
sented in Plate IV. Fig. 31, shows no trace whatever of a cavity ; while 
Figure 28, also a twenty-four-cell stage, shows at the animal pole a 
number of lacune or intercellular vacuoles. between the cells of the 
