42 FRANKLIN PARADISE JOHNSON 



regard to either of the two swellings that their boundaries corre- 

 spond absolutely to the boundaries of the two above-named 

 zones. 



The first evidence of the development of folds in the bulbus 

 analis was found in an embryo of 22.8 mm. At .30 mm, three 

 distinct folds are present, the outfoldings of the epithelium occu- 

 pying positions posteriorly, and on the right and left sides. These 

 three primary folds are present throughout stages up to birth. 

 Whether they are present in post-natal rectums has not been 

 determined. They extend throughout the whole length of the 

 bulbus analis. Below they terminate by ending on the zona 

 intermedia. This, in stages up to about 30 mm., is flattened 

 ventro-dorsally. The right and left primary folds become di- 

 rectly continuous with the right and left borders of the bulbus 

 terminalis respectively. In stages of 31 mm. and over, the bul- 

 bus terminalis or zona intermedia is flattened laterally. This 

 band-like portion of the rectum, therefore, either undergoes a 

 rapid change in its form or in its position. Just what happens at 

 this stage was not clearly brought out by the embryos examined. 

 From the position occupied by the left fold, in the later stages, a 

 twisting to the right through an angle of 90 degrees seems to have 

 taken place. However, from the arrangement of the other two 

 primary folds and the tissues in the immediate neighborhood of 

 epithelial tube, it is not probable that any such twisting has gone 

 on. 



In all the later stages of development the constancy with which 

 the primary folds are found is very striking. Moreover, these 

 folds terminate at their lower extremites in a similar manner .in 

 almost all stages examined. The outfolding on the left side 

 passes up onto the ventral border of the laterally flattened zona 

 intermedia; the posterior fold extends directly into the dorsal 

 border of the same ; while the right fold extends as far as the zona 

 intermedia and terminates on its right side, about half way be- 

 tween its ventral and dorsal borders. 



In an embryo of 37 mm. secondary folds begin to appear on the 

 bulbus analis. These are longitudinal folds similar to, but 

 smaller than, the primary folds. They develop rapidly and soon 



