DEVELOPMENT OF THE RECTUM 41 



up to birth, although its form changes greatl}^ At first it is elon- 

 gated. It increases in size proportionally up to the 37 mm. 

 stage. Little or no increase in length occurs throughout stages 

 42, 44.3 and 65 mm., although the increase in its cross diam- 

 eter is considerable. From this stage on there is an increase in 

 the length of the swelling, but the direction of greatest growth is 

 transversely. Whereas in the earlier stages its length is several 

 times its diameter, at birth the diameter slightly exceeds its 

 length. 



In embryos of 13.6 and 16 mm. there appears below the above- 

 described spindle-shaped swelling a second swelling. This is 

 much shorter, and not so well marked as the upper. It exists as 

 a swelling only for a short period of embryonic life. In an em- 

 bryo of 29 mm. the swelling has largely disappeared, while in 

 embryos of 30 mm. and over it is represented as a narrow slit- 

 like cavity. 



Keibel and Lewis have described these swellings of the rectal 

 tube in corresponding stages of development. The upper of 

 the two swellings, which I have termed the 'bulbus analis,' occu- 

 pies a position in the embryo which corresponds quite closely to 

 that occupied by the ampulla recti of the adult. However, as has 

 been shown, it does not form the adult rectal ampulla. The 

 walls of the embryonic swelling become invaginated and form 

 numerous longitudinal .folds and ridges. The study of these 

 folds throughout successive stages of development show that they 

 are the rectal columns. As these are found principally within 

 the zona columnaris of the pars analis recti, it follows that the 

 spindle-shaped swelling of the embryo becomes in large part the 

 zona columnaris of the adult. The upper portion of the bulbus 

 analis, however, lies above the ano-rectal line, and from the 

 standpoint of histology, belongs to the ampulla recti. The lower 

 swelling, which I have termed the 'bulbus terminalis,' develops 

 from the dorsal portion of the cloaca which Tourneux has called 

 the 'vestibule anal.' It is limited above by the constriction 

 between it and the bulbus analis, and below by the anal mem- 

 brane. It forms the lower portion of the pars analis recti, namely 

 the zona intermedia. It cannot be definitely said, however, in 



