206 
E, A. SCHAFER,. 
slight as to be almost imperceptible, as in the Holothuriaii 
blastosphere, or it may be more obvious as in that of the 
Sponge and Paludina. And every transition is found between 
these cases and those in which the amount of pabulum is so 
great in proportion that it can no longer be contained in the 
typical single layer of cells, but becomes accumulated in the 
cavity of the sphere as well, either still enclosed in definite cells 
as in the Amphibian, or for the most part unenclosed in cells as 
in the Pish, the Bird, and the Crustacean. 
The next most important change that occurs in the typical 
unencumbered ovum is the invagination of one part of the simple 
wall of the blastosphere, in such a manner as to convert the 
hollow single-layered vesicle into a cup with double walls. The 
cleavage- cavity or cavity of the blastosphere is concomitantly 
reduced in size or even obliterated as it is encroached upon by the 
invaginated part of the wall, and its place is taken by the cavity 
of the cup. The orifice of this, at first widely open, becomes 
gradually closed up by the continued growth of the two layers 
at the mouth of the cup, until but a narrow aperture remains, 
which persists for a time, but at length in many cases becomes 
closed. This aperture is termed by Haeckel the primitive mouth 
or protostoma, by Lankester the blastopore. 
The result of the completion of this process of invagination is 
again a hollow vesicle, but its walls are now composed of two 
layers of cells instead of one. The inner or invaginated layer is 
termed entoderm and the outer or enclosing layer ecto- 
derm.’^ The cavity which they enclose always becomes the 
future alimentary cavity, and hence the name gastrula ” has 
been applied by Haeckel to this stage of development. We may 
also speak of it as the cup^ stage or as the stage of the hilaminar 
blastoderm. 
In typical ova the original blastosphere is uniform through- 
out, and it would be impossible to point out the part of the wall 
which is to be invaginated. But in those ova which have even a 
small excess of nutritive material and in which therefore, as 
already mentioned, some of the cells of the blastosphere are 
characterised by containing this excess, we always find that it is 
this particular part of the wall of the vesicle which is involuted 
to form the inner layer of the cup. So that even in the stage of 
the blastosphere we can predict which cells are to become ento- 
derm and which ectoderm. 
Finally, in those ova in which the nutritive material largely 
}>repon derates we find that even when the cavity of the blasto- 
sphere is not entirely filled up by that material (and of course 
also when it is so filled up), the mechanical hindrance to any 
invagination is so great that the process cannot be e|Fected» 
