DEVELOPMENT OF THE RECTUM 17 



the term 'ampulla recti' is wholly inapplicable, as will be shown. 

 At the time his article was written, Lewis realized the importance 

 of further study upon this subject, and pointed out to me the 

 desirability of following this swellinj^- throughout the later stages 

 of the embryo. 



The upper swelling persists as a definite swelling throughout 

 stages up to birth. It is an embryonic structure quite similar to 

 that found at the junction of the ileum and the colon, which the 

 author ('13) designated by the term 'ampulla coli.' Originally it 

 was likewise intended to use the term 'ampulla recti' for this 

 embryonic structure in the rectum. From its position in the 

 early stages of the embryo one would suppose that it was iden- 

 tical with the ami:)ulla recti of the adult. However, such is not 

 the case. With the growth of the embryo it becomes relatively 

 shorter and shorter and gradually descends lower and lower in the 

 pelvis. It eventually turns out to be a portion of the pars analis 

 recti of the adult, principally the zona columnaris. As this 

 structure has as yet received no definite name, the author pro- 

 poses the term 'bulbus analis.' 



With regard to the lower swelling it can be said that it forms 

 the zona intermedia of the adult pars analis recti, although noth- 

 ing can be definitely said with respect to its boundaries. It per- 

 sists as a swelling only throughout the early stages. 



The dorsal portion of the cloaca into which the rectum opens 

 has been termed by Tourneux ('90) the 'vestibule anal.' In 

 describing it he says: 



L'extremite inferieure du rectum debouche maintenant dans un 

 vestibule qui se prolonge en avant jusqu'au l:»ouchon cloacal, et qui 

 represente le sommet meme de I'anse cloacal unissant primitivement le 

 rectum au bouchon. Nous donnerons a ce vestibule le nom de vestibule 

 anal, et a sa parol inferieure ou cutanee exclusivement formee de cellules 

 ('pitheliales, le nom do UK^mhrane vestibulaire ou de membrane anale. 



When the cloaca becomes divided into dorsal and ventral j^arts, 

 whether it he by means of the downgrowth of the perineal fold 

 (as described by Tourneux) or b}^ means of a pushing together of 

 the two lateral mesodermic folds (Retterer '90), it is evident that 

 the dorsal half of the cloaca becomes incorporated with the rectum 



THE AMERICAN JOfRXAL OV AXATOMV, VOL. 10, NO. 1 



