18 FRANKLIN PARADISE JOHNSON 



and forms its lower portion. It is also evident that the lower 

 swelling' described in the 16 mm. and 19 mm. embryos, is formed 

 from the old cloaca. This is what is taking place in the above- 

 described 13.6 mm. embryo, where the lower bulb is only par- 

 tially formed. This conclusion seems justifiable not only because 

 the constriction between the bul]:)us analis and the cloaca in the 

 early stages is identical with the constriction between the bulbus 

 analis and the lower swelling in the later stages, forming a definite 

 boundary superiorly, but because of the constant and definite rela- 

 tion presented by the anal membrane. This membrane, which is 

 identical with the posterior half of the cloacal membrane, forms 

 the inferior wall of both the anal vestil)ule in the early stages and 

 of the lower swelling in the later stages. The lower swelling, 

 which develops from a part of the cloaca (Tourneux's 'vestibule 

 anal') and which forms the zona intermedia of the adult pars 

 analis recti, will be sj^joken of by the term 'bulbus terminalis.'^ 



Late stages 



The form of the rectum of an embryo at 30 mm. (H.E.C. 913) 

 is shown in figures 8 and 12. It is to be noted that the figures 

 represent the basal surfaces of the epithelium, that is, those sur- 

 faces which are covered by mesenchyma. The grooves, there- 

 fore, which are seen on the outer surface are to be considered as 

 folds of the mucous membrane turned into the lumen of the rec- 

 tum. For clearness of description in the remainder of this paper, 

 these folds of mucous membrane into the lumen will always be 

 spoken of as 'in-foldings,' while the furrows in between them will 

 be called 'out-foldings.' The side view shows that the dorso- 

 ventral curxature is slightly greater than at 22. S nnn. The 

 l)ulbus analis is not well marked off from the intestinal tube above, 

 but below, where it joins the buUnis terminalis there is a great 

 diminution in its dorso-venti'al thickness. The bulbus termin- 



1 Since the 'bulbus aiuiUs', (he 'bulbus terniinalis,' and the swelling at the 

 ileo-colic junction aic all similar embryonic- structures, for the sake of consist- 

 ency, it would be better perhaps, to designate the latter liy the term 'bulbus 

 coli' rather than 'ampulla coli.' 



