23 
descendants of one common but very remote ancestral form, 
whose body consisted of a simple two-walled vase, with a 
central digestive cavity opening externally at one end of the 
body. 
Haeckel, who is the originator and leading advocate of this 
hypothesis, has proposed to call this ancestral form a “ Gas- 
traea;” and the gastrula stage of development he regards as 
a trace or indication of this distant ancestry, which is still 
retained and passed through during the early stages of the 
development of animals which are now very widely separated. 
The gastrula theory cannot be regarded as one of the es- 
tablished generalizations of science, and the evidence which 
has so far been accumulated by embryologists is not by any 
means straightforward or satisfactory. The theory is one of 
the most interesting embryological problems under discussion, 
however, and any new information which bears upon it is 
of value. 
The fact that the oyster goes through a very well marked 
and very slightly modified gastrula stage is therefore of great 
theoretical interest, and more so since Salensky, a distin- 
guished Russian embryologist, has proposed in place of the 
gastrula theory another theory, which is based, in part, 
upon erroneous observations upon the development of the 
oyster, which Salensky says does not pass through the gas- 
trula stage of development at all, but forms a digestive cavity 
in another way. 
The edges of the primitive mouth of the oyster continue 
to approach each other, and finally meet and unite, thus clos- 
ing up the opening, as shown in Figure 36, and leaving 
the digestive tract without any communication with the 
outside of the body, and entirely surrounded by the outer 
layer. The embryo shown in Figures 32 and 36 are rep- 
resented with the dorsal surface below, in order to facili- 
tate comparison with the adult, but in Figure 37, and most 
of the following figures, the dorsal surface is uppermost, for 
more ready comparison with the adult. The furrow in which 
the primitive mouth was placed still persists, and soon a small 
irregular plate makes its appearance at each end of it. These 
