22 FRANKLIN PARADISE JOHNSON 



analis has taken place by an outward expansion alone, there 

 being practically no lengthening of this portion of the rectum. 



In embryos of 70 mm. and 88 mm. similar relations with regard 

 to folds are seen. The numl^er of secondary folds is only slightly 

 greater than at 65 mm. 



Longitudinal sections of the 88 mm. stage show several new 

 points with regard to the epithelium and the outer coats. The 

 transition from the simple columnar' epithelium of the ampulla to 

 the stratified squamous epithelium of the skin is not effected by 

 a continuously gradual transition but takes place in three clearly 

 defined stages. The epithelium of the ampulla is simple columnar 

 containing many goblet cells. For the most part the bul}:)us 

 analis is lined with an epithelium of two to three layers of colum- 

 nar cells, the basal of which are cuboidal and have deeply staining 

 nuclei. The protoplasm of the cells is distinctly granular. For 

 a short distance below the beginning of this epithelium are found 

 a few scattered goblet cells similar to those in the epithelium 

 above. The transition between this epithelium and that of the 

 ampulla takes place rather gradually at the ano-rectal line. This 

 line, however, does not correspond in position to the constriction 

 between the bulbus analis and rectal ampulla, as will be shown 

 later. 



The epithelium of the bulbus terminalis, or zona intermedia as 

 it will hereafter be called, is composed of five to ten layers of 

 polygonal cells, and differs only from that of the skin of the nates 

 in thickness. The transition between this and that of the zona 

 columnaris is again rather gradual. It forms the linea sinuosa 

 analis. The zona intermedia passes directly over into the zona 

 cutanea, the line of division between the two, the ano-cutaneous 

 line, not being developed at this stage. The muscularis mucosae 

 in the region of the ampulla is distinct but its few fibers dwindle 

 out in the region of the bulbus analis. 



Practically the same relations are seen in an embryo of 99 mm. 

 The portion of the circular muscle layer which develops into the 

 internal sphincter shows a slight thickening. 



In figure 25 is represented one-half of the rectum of a 110 nun. 

 embryo viewed from the inside. The figure was drawn directly 



