22 
which is distinct from the outer wall of the body. Around ' 
the primitive mouth these two layers are continuous with, 
each other. 
The w’ay in which this cavity, with its wall and external 
opening, has been formed, will be understood by a compari- 
son of Figure 33, with Figure 23. The layer which is 
below in Figure 28 has been pushed upwards in such a way as 
to convert it into a long tube, and at the same time the outer 
layer has grown downwards and inwards around it, and 
has thus constricted the opening. The layer of cells which 
is below in Figure 28 thus becomes converted into the walls 
of the digestive tract, and the space which is outside and be- 
low the embryo, in Figure 28, becomes converted into an in- 
closed digestive cavity, which opens externally by the primi- 
tive mouth. 
This stage of development, in which the embryo consists of 
two layers, an inner layer surrounding a cavity which opens 
externally by a mouth-like opening, and an outer layer, which 
is continuous with the inner around the margins of the open- 
ing, is of very frequent occurrence, and it has been found, , 
with modifications, in the most widely separated groups of 
animals, such as the star-fish, the oyster and the frog, and 
some representatives of all the larger groups of animals, except 
the Protozoa, appear to pass during their development through 
a form which may be regarded as a more or less considerable 
modification of that presented by our oyster embryo. This 
stage of development is known as the gastruia stage. 
Certain full grown animals, such as the fresh water hydra 
and some sponges, are little more than modified gastrulas. The 
body is a simple vase, with an opening at one end communi- 
cating wfith a digestive cavity, the wall of which is formed by 
a layer of cells, which is continuous around the opening with 
a second layer which forms the outer wall of the body. This 
fact, together wfith the fact that animals of the most wfidely 
separated groups pass through a gastruia stage of develop- 
ment, has lead certain naturalists to a generalization, wfiiich is 
known as the “ gastruia theory.” This theory or hypothesis 
is that all animals, except the Protozoa, are more or less direct 
