20 FRANKLIN PARADISE JOHNSON 



The epithelial tube of the rectum of the embryo of 29 mm, is 

 shown in figures 9 and 13. It will be observed that the three 

 infoldings are well marked, though they are somewhat less devel- 

 oped than in the above-described 30 mm. stage. These infold- 

 ings extend themselves throughout the full extent of the bulbus 

 analis and terminate below on the bulbus terminalis. Above 

 they terminate (with the exception of the ventral one) at the 

 upper limit of the swelling. The ventral infolding extends beyond 

 the bulbus analis. It should also be noted that whereas before 

 the bulbus terminalis was flattened dorso-ventrally, it is now 

 flattened laterally. Moreover, a distinct swelling in this region 

 can hardly be said to exist. 



The outfoldings of the epithelial tube demand especial atten- 

 tion. It is to be noted that the outfolding forming the left border 

 of the rectum, can be traced into the upper border of the flattened 

 bulbus terminalis. The one on the right terminates on the right 

 side of the bulbus terminalis, while the dorsal passes down direct- 

 ly into the dorsal wall of the same. These conditions suggest 

 that the epithelial tube in its lower part has been twisted through 

 an angle of 90"^ in a direction which, when looked at from above, 

 is clockwise. Whether any such twisting does take place between 

 the two last described stages is a rather difficult problem to deter- 

 mine. More will be said in regard to this below. 



Although the bulbus terminalis shows again as a flattened slit- 

 like canal in embryos of 30 mm. (H. 108) and 31 mm. (H. 57), 

 slight A^ariations are met with in regard to the folds, particularly 

 at the lower end of the zona columnaris. In the 30 mm. stage is 

 found a small additional ventral outfolding arising from about the 

 middle of the ventral infolding. At 31 mm. the left outfolding 

 does not quite reach the upper border of the bulbus terminalis but 

 ends on the side of it near the upper border. 



In an embryo 37 mm. (H.E.C. 820) the epithelial tube of the 

 rectum presents a number of long longitudinal folds (figs. 10 and 

 14). Examination of the model of this rectum shows distinctly 

 that the three outfoldings described in earlier stages are still pres- 

 ent, although their identity is somewhat obscured by the presence 

 of additional secondary folds. As in the 29 mm. stage the out- 



