62 MORGAN. [Vou. IX. 
and inasmuch as no direct communication with the exterior 
exists the fluid must either exude mechanically through the 
walls or else be taken up by the cells and passed out. During 
this time the fluid becomes of firmer consistency, if one may 
judge from preserved material. Part of the decrease in size of 
the blastocoel space may be accounted for by the bulging out of 
the posterior plate, but only a relatively small amount of the 
original volume can be accounted for in this way. The body- 
cavities are enlarging during this period and as the cavities are 
completely cut off from the cavity of the blastocoel a transu- 
dation of fluid must take place into the interior of each body- 
cavity. 
There are many problems of great interest from a mechanical 
point of view, in connection with the relations existing between 
the fluids filling a space bounded by cells and the cells them- 
selves. I had my attention called to this problem for the first 
time when reading Brauer’s paper on the development of 
Hydra. The egg is exceedingly large, but the blastula derived 
from the egg scarcely larger than the egg itself. All of the 
protoplasm of blastula is gathered into a narrow peripheral 
shell, and the center of the sphere is then filled with a large 
fluid space. The question suggested itself as to whether all of 
the protoplasm of the egg could be represented in the per- 
ipheral shell of protoplasm. It seemed impossible that it could 
be so represented, and I made an estimate to determine how 
wide such a shell of a sphere would have to be to receive all 
the protoplasm of the egg—it being understood that a slight 
increase in size had actually taken place in the embryo. Only 
approximate measurements could be made in the case cited 
above, but such as they were they showed that the slight 
increase in the diameter of the sphere was amply sufficient to 
accommodate in a peripheral shell all the protoplasm of the egg 
—a sphere with a slightly smaller diameter. And the increase 
in diameter calculated as necessary a préort corresponded with 
_ the actual measurements of the figures themselves. The fluid 
in the center of the blastula must have passed ‘through or 
between the peripheral cells. But although it was possible to 
account in this way for all of the protoplasm of the egg, it 
