DEVELOPMENT OF THE LARGE INTESTINE 191 



in the umbilical cord, pointing away from the embryo. Large 

 at its base where it joins the colic ampulla, the vermiform process 

 tapers gradually towards its bhnd end. The before-mentioned 

 narrow portion of the large intestine, now 0.09 mm. in diame- 

 ter, has increased much in length. The lumen of the lower end 

 of the digestive tube no longer leads into the cloaca, but opens 

 to the outside by an extremely small aperture. 



In the further course of its development, the swelling in the 

 rectal region becomes much larger, and longitudinal folds make 

 their appearance. These longitudinal folds increase in numbers, 

 and are markedly constant in position in all the older stages. 

 Just what is the fate of these folds I am unable at the present 

 time to state precisely, but it is not improbable that they give 

 rise to the rectal columns (columnae rectales Morgagni) while 

 the spaces between them no doubt develop into the rectal sinuses 

 (sinus rectales) . A discussion of the further development of this 

 portion of the digestive tract, however, has been omitted from 

 the present paper. 



The development of longitudinal folds 



In an embryo of 22.8 mm. one sees for the first time a change 

 in the form of the epithelium. In the colic ampulla, which now 

 has a diameter of 0.20 mm., the epithelium shows three low 

 longitudinal ridges on its inner surface. These ridges also 

 extend for short distances into the colon and vermiform process. 



It becomes necessary at this point to explain the manner in 

 which the terms 'ridges' and 'folds' have been used through- 

 out the remainder of this article. The term 'ridge' has been 

 employed to designate a thickening of the epithelium which 

 projects into the lumen. It must have no corresponding inden- 

 tation on its under surface into which mesenchyma would ex- 

 tend. By a 'fold' is meant a projection with an indented basal 

 surface, into which the underlying mesenchyma protrudes. This 

 distinction is desirable, as its usage makes it possible to explain 

 in few words the shape of the basal surface of the epithelium 

 along with that of its free surface. 



