DEVELOPMENT OF THE RECTUM 19 



alis itself is only slightly thicker than the constriction. Viewed 

 from in front the upper extremity of the bnlbus analis is again 

 indistinctly marked, whilst below its lower boundary is quite 

 sharp. The whole surface of the anterior wall of the bulbus 

 analis is deeply furrowed by a single longitudinal groo\'e. Two 

 other well mai'ked grooves are found, one on either side. 



Both the internal circular and the outer longitudinal muscle 

 coats of the rectum are distinct; also the levator ani and external 

 sphincter muscles are apparent. The circular layer of the muscu- 

 laris of the rectum extends dow^n to the constriction between the 

 swellings of the two zones, but displays no special thickening which 

 characterizes this region in later stages. The outer longitudi- 

 nal muscle coat is still faintly visible, but its fibers can be seen 

 running beyond the region of the circular coat. They soon be- 

 come lost in the mesenchyma. The levator ani is seen running an 

 oblique coiu'se from abo\^e downward and toward the rectum. 

 The extei-nal si:)hineter lies just below its inferior extremity and 

 its fibers are not distinctly" marked off from those of the levator 

 ani. Numerous nerve fibers and small blood vessels are seen in 

 the substance of and between the muscle layers. 



The epithelium is different in different regions. In the bulbus 

 analis it is 0.036 to 0.045 nuu. in thickness and has two to three 

 layers of oval shaped nuclei. C'ell boundaries are not distinct. 

 In the region of the anal opening the epithelium is of the same 

 thickness, but the cells of the epithelium are polj^gonal in shape, 

 and have distinct boundaries. Of the four to five layers of cells, 

 the uppermost are somewhat flattened. In the region of and 

 that adjoining the bull)us tei-minalis the epitheUum is only 0.024 

 to 0.030 mm. At the lower extremity of the Inilbus terminalis is 

 the transition between the two tyi)es of epithelia described above. 

 The lumen is pervious throughout, a slight opening being present 

 through the anal membrane. 



As regards the degree of development of the rectum, the embryo 

 just described pro))ably represents a retarded stage. This is made 

 evident because in three other embryos of about the same age 

 which were studied— 29 mm. (H.E.C. 914), 30 mm. (H. 108), 31 

 mm. (H. 57) — more advanced conditions were found. 



