DEVELOPMENT OF THE RECTUM 47 



the anal papillae (slight elevations on the outside of the anus). 

 He detects the anlagen of the fibers of this muscle in an embryo 

 of 12.5 mm. 



Alention should be made regarding the extent and relations of 

 the various muscles to the different parts of the pars analis recti. 

 In the early stages, both the circular and longitudinal layers of 

 the muscularis terminate at the constriction between the bulbus 

 analis and the bulbus terminalis. In the later embryonic stages 

 the circular muscle extends well into the region of the zona inter- 

 media, while the longitudinal layer extending not quite so far 

 down, terminates just above the external sphincter muscle. The 

 external sphincter muscle surrounds the middle part of the zona 

 intermedia. 



The development of the plicae transversales recti may be 

 summarized in few words. They apparently develop after the 

 manner of the folds of the large and small intestines, that is, by 

 a gradual infolding of the mucous membrane. They were first 

 seen in an embryo of 99 mm. From this stage on they become 

 more pronounced. Gradually they spread further and further 

 apart with the growth of the rectum as a whole. 



A study of the developmental topography of the pars analis 

 recti shows that it forms a relatively much greater portion of the 

 digestive tube in the embryo than in the adult. Its growth in 

 length, therefore, is proportionally less than its growth in diame- 

 ter. With this relative shortening, the pars analis recti grad- 

 ually descends in the pelvis. Whereas in the early stages the 

 whole of the pars analis recti lies between the thu'd sacral verte- 

 bra and the tip of the coccyx, at birth it lies completely below the 

 tip of the coccyx. 



In the early stages of the embryo the meeting point of the ento- 

 derm and the ectoderm lies at the lower end of the bulbus termin- 

 alis at the anal membrane. Since it has been shown that the 

 bulbus terminalis develops into the zona intermedia, it follows 

 that at birth and in the adult the limit between the two germ 

 layers must lie somewhere near the lower boundar}^ of this zone, 

 that is, near the ano-cutaneous line. 



