40 FRANKLIN PARADISE JOHNSON 



derm, the line of separation between the ectoderm and entoderm 

 being represented by the ano-rectal Une. His reason for main- 

 taining this view was because of the change in the character of the 

 epithehum at this point. The few isolated intestinal glands 

 Cglandes erratiques') and goblet cells he regards as fragments 

 of entodermic epithelium which have descended into the ecto- 

 derm of the anal region. 



After the breaking down of the anal membrane, there is no 

 apparent line of demarcation between the entoderm and the 

 ectoderm. However, when the cloacal or anal membranes are 

 present, the junction of the two germ layers is definitely marked. 

 This junction lies just below the bulbus terminalis. Since 

 both the bulbus analis and the bulbus terminalis lie above the 

 anal membrane they are composed of entoderm and it is evident, 

 therefore, that the derivatives of these, the zona columnaris and 

 the zona intermedia of the pars analis recti, are of entodermal 

 origin. The exact line of demarcation in the adult cannot be 

 definitely located, but it is undoubtedly somewhere in the neigh- 

 borhood of the linea ano-cutanea. 



DISCUSSION AND SUMMARY 



A study of a few young stages of the human embrj^o confirmed 

 the results of Keibel and Pohlman in that the cloaca in the human 

 embryo is never open to the outside, and that division of the cloaca 

 into urogenital sinus and rectum takes place in embryos of about 16 

 mm. in length. After separation has taken place, the posterior por- 

 tion of the cloacal membrane persists as the anal membrane and 

 occludes the rectum at its lower extremity. It is present for some 

 little time and exists in the form of a few irregular cells similar to 

 those of the wall of the anus. This was last observed in an 

 embryo of 22.8 mm. An exception to this is found in the 16 nmi. 

 embryo, in which the anal canal is alreadj^ open to the outside. 

 Keibel ('08), however, shows that ordinarily the anus is closed in 

 embryos as old as 26 mm., while Broman ('11) states that its 

 lumen becomes patent in embryos of about 33 mm. 



In embryos of 7 and 10 mm. the rectal tube presents a spindle- 

 shaped swelling. It is apparent as a swelling throughout stages 



