84 BULLETIN OF THE 
its anterior surface. These facts afford strong evidence that this portion 
of the alimentary tract is derived from the entoderm rather than from the 
stomodeeal infolding of the ectoderm. The cellular elements of which it is 
composed do not, however, differ enough from those of the stomodeum 
to add anything to the reasons just given for supposing an entodermic 
origin. But if, as I believe, this is not an outgrowth of the stomodeum, 
it must be the first-formed portion of the mesenteron, the walls of which, 
as we shall subsequently see, are begun at both ends and completed by 
the gradual advance and ultimate meeting of the two separate formations. 
On each side of the stomach are given off ceca, which extend into the 
bases of the limbs. The cellular elements composing the walls of these 
tubes are flattened. 
The walls of the anterior or stomodzal portion of the alimentary canal 
are composed of three layers: the cuticular, the epithelial, and the peri- 
toneal. The pharynx, the cesophagus, and the sucking stomach are all 
lined with a cuticular layer which is continuous at the mouth with the 
cuticular covering of the body. In the pharynx it is thickened and cor- 
rugated by tooth-like projections, but in the cesophagus and the stomach 
it is much thinner and not roughened. I have not been able to make 
out satisfactorily whether this layer extends into the post-gastric portion 
or not. If it does, this would be an argument in favor of the ectodermic 
origin of this portion of the canal. 
The posterior part of the alimentary canal— the proctodeum — does 
not begin until the reversion of the embryo is well advanced. Its exter- 
nal orifice is minute and leads directly into an expanded portion, which 
becomes the stercoral pocket. This enlargement is present at an early 
stage of the invagination, and presents in sagittal section a triangular 
outline (Pl. VIII. fig. 54). Its walls are at this time thick and com- 
posed of large ectodermic cells, which are, however, only a single layer 
deep. The invagination forming the proctodzum pushes before it an 
enveloping layer of the already formed mesoderm. The invagination is 
gradually differentiated into two parts: a straight narrow tube (the ter- 
minal portion of the rectum), and the stercoral pocket. The wall of the 
proctodeum is composed of columnar epithelium, the large spherical 
nuclei of which are placed close to the inner ends of the cells, which 
almost meet, and thereby nearly obliterate the lumen of the tube. This 
epithelium is enveloped externally by the usual layer of flattened meso- 
dermic cells. The stercoral pocket increases rapidly in size, and becomes 
pear-shaped in outline. The cellular elements of its walls change from a 
columnar to a flattened epithelium. At the time of hatching its wall is 
