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 

 stomodseal infolding of the ectoderm. The cellular elements of which it is 

 composed do not, however, differ enough from those of the stomodseum 

 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 storaodgeum, 

 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 cseca, 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 stomodseal portion of the alimentary canal 

 are composed of three layers : the cuticular, the epitlielial, and the peri- 

 toneal. The pharynx, the oesophagus, 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 tootli-like projections, but in the oesophagus and the stomach 

 it is much thinner and not roughened. I have not been able to make 

 out satisfactorily wliether 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 proctodaeum — does 

 not begin until the reversit)n 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 (PI. 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 proctodaeum 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 

 proctodaeum is composed of columnar epithehum, 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 



