82 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
cavity may attain a volume equivalent to one half or two thirds that of 
the undivided ovum. Throughout its whole history, from its inception 
to its culmination, both in the living egg and in preserved material, 
the cavity is always sharply marked off from the protoplasm of both 
cells. This is true, no matter what may be the point of view from 
which the egg is observed. Of course the boundaries of the cavity aro 
indistinet where they lie oblique to the optical axis of the microscope, 
but there is always one focal plane at which the limiting “membrane” 
of the bounding cells sharply and distinotly separates the protoplasm 
from the fluid contents of the cavity, and moreover there is no trace 
whatever of any shading off of the protoplasm toward the cavity such as 
Stauffacher (93) describes in Cyclas. We are therefore compelled to 
conclude that the cavity is distinctly intercellular. 
The maximum development of the cavity is followed by a forcible 
expulsion of its contents. "This takes placo suddenly, and tho elimina- 
tion of the fluid may be total, or only partial. After a total elimination 
the egg resumes its original size, and tends to take again the spherical 
form. In case of a partial expulsion the cavity retains its polar diameter, 
but the antero-posterior diameter is reduced (Plato I. Figs. 5, 6). In the 
egg represented in Figure E, the spindles leading to the third generation 
were present when the cavity of the two-cell stage had reached its max- 
imum. It often happens that two or even more expulsions of the liquid 
contents of the cavity occur between the two- and the four-cell stages. 
In the series represented in Plate I. Figs. 1-7, drawn from the same egg 
kept under continuous observation, the two blastomeres had attained the 
maximum divergent or rounded condition at 11.30 a. m. At 12.35 P. M. 
they had reached the flattened or coalescent, condition (Figs. 1, 2), and 
at 19.45 v. m. the central cavity had appeared (Fig. 2). This increased 
in size (Figs. 3-5), reaching a maximum at 1.90 p.m., when a partial 
expulsion occurred (Fig. 6). After this expulsion the nuclei could no 
longer be seen distinctly in the living egg. The cavity again increased 
in volume (Figs. 6, 7), and at 9.02 p. m. a second and total elimination 
took place coincidently with the division into four cells; this was accom- 
plished, i. e. the furrows had reached the vegetative pole, at 2.05 P. m. 
It is not always possible to determine the point at which the fluid 
contents are ejected. When a sudden reduction in the size of the cavity 
is noticed, there is sometimes visible in the albumen adjacent to the 
cleavage furrow a small sphere of transparent matter differing in its 
refractivo index from the surrounding albumen into which it very quickly 
merges. When, however, the reduction in the size of the cavity is 
