DEVELOPMENT OF THE RECTUM 21 



folding on the left side passes down onto the ventral border of the 

 flattened bullus terniinalis, the right terminates on the right side 

 of the bulbus terniinalis about half way between the dorsal and 

 ventral borders, while the posterior outfolding forms the dorsal 

 border. Both the right and posterior oiitfoldings are divided 

 into secondary folds. Just abo\'e the bulbus analis five to six 

 irregular folds are seen which undoubtedly represent the j^re- 

 villous folds of the ampulla recti. Regarding the outer layers of 

 the rectum, but few changes have taken place. The outer longi- 

 tudinal coat is more distinct than before and is pierced by numer- 

 ous small groups of nerve fibers, particularly along its dorsal wall. 

 The external sphincter muscle is seen as a distinct broad band of 

 fibers entirely encircling the lower portion of the rectum. It lies 

 just below the fibers of the levator ani. 



As seen in figures 15 and 17, the epithelial tube of the pars 

 analis recti in an embryo of 44.3 mm. (H.E.C, 1611) presents a 

 more advanced condition with regard to folds. Although more 

 secondary folds are present, the primary folds are distinguishable 

 and present similar relations to those of the 37 mm. embryo. 

 Both primary and secondary folds show distinctly in a series of 

 cross sections of the rectum of a 58 mm. embryo. The primary 

 folds are similar in position and relation to those of the former 

 stage. The secondary folds are more numerous, the right pri- 

 mary fold being divided up into six secondary folds, the left into 

 seven, and the dorsal into eight. New secondary folds are seen 

 beginning in between those already present. The}' de\'elop 

 largely after the manner of villi in the small intestine, by first a 

 thickening of the epithelium and then an invagination of the 

 epithelium along the thickened ridge into the lumen. 



A model of the pars analis recti at 65 mm. (H. 55) is shown in 

 figures 16 and IS. With the exception of a few additional second- 

 ary folds, the same picture is i:)resented as before. The right 

 primary is made up of five secondary folds, the left of seven, and 

 the dorsal which is much the largest, is composed of about ten 

 secondary folds. Comparison between figures 10, 15, and 16 and 

 figures 14, 17, and 18 will show that the growth of the bulbus 



