DEVELOPMENT OF THE KECTUM 43 



obscure the i)rimary folds. At 54 nun. the secondary folds are 

 quite nuiiKM-ous. Alost of them are formed in an embryo of 135 

 mm., but an embryo of 240 mm. shows some increase in their 

 numbers. Apparently none are formed after this stage. 



The significance of these longitudinal folds of the bulbus an- 

 alis cannot be questioned. The infoldings of the mucosa form 

 the columnae rectales (columns of Morgagni). When viewed 

 from the inside (fig. 25) the i)rimary folds are not distin- 

 guishable and only the larger columns are apparent. Numerous 

 small secondary folds lie at the bottom of the outfoldings or 

 sinuses. The number of such vertical folds or columns is large, 

 there being as many as 60 in a 240 mm. embryo. The smaller of 

 these folds, so far as general structure is concerned, do not dift'er 

 from the larger columns and should, therefore, be considered as 

 the equivalents of the usually-described rectal column. 



The rectal sinuses (sinus rectales) of the adult, as they are 

 usually described, are found at the lower extremities of the inter- 

 columnar clefts. An examination of models shows that such pock- 

 ets are present as the lower portions of the outfoldings, which in 

 places extend down below the general level of the mouth or open- 

 ing of the cleft. 



The anal valves are found at the lower limits of the folds. In 

 the embryo these do not have so much the appearance of the 

 aortic valves as is usually figured for the adult, but are narrower 

 and thicker. It seems reasonable to the author that distention 

 must pla}^ an important part, in the further formation of the 

 valves and sinuses. The great variation in number, size, and 

 and shape in the same and in difTerent individuals, and the extent 

 to which the pars analis later becomes distended, would seem to 

 be evidence in support of this view. 



The histology of the rectum throughout the different periods 

 of development may next be briefly reviewed and summarized. 

 The first part of the rectum including the ampulla is similar in 

 its development to the colon as has been previously described 

 (Johnson '13). 



In the earlier stages of development the pars analis recti is 

 lined by an epithelium composed of two to three layers of low 



