THE ALIMENTAEY CANAL. 549 



During the fifth week the cloaca, which up to this time has 

 been a single dilated chamber (Fig. 233), becomes divided, by 

 the growth backwards of a septum from the angle between the 

 allaiitoic stalk and the intestine, into two separate tubes ; of 

 these, the dorsal one (Fig. 234, C) is continuous with the 

 intestine and forms the rectum ; while the ventral one, H&, 

 receives the allantoic stalk, and the Wolffian ducts and ureters, 

 and forms the urino-genital passage. 



The septum which thus divides the cloaca into rectal and 

 urino-genital chambers is formed by the union in the median 

 plane of two lateral folds or ridges, which arise from its sides ; 

 it reaches the surface of the body just below the root of the tail, 

 about the end of the fifth week (Fig. 234). The proctodssal 

 opening is formed about the same time, but it is not certain 

 whether this takes place before or after the completion of the 

 septum : in the former case there would be for a short time a 

 single cloacal aperture ; in the latter case the rectal and urino- 

 genital apertures would be distinct from the first. 



The later stages in the development of the part of the 

 alimentary canal from the oesophagus to the rectum present 

 few features of special interest. The epithelium lining the 

 oesophagus is ciliated during the fifth and sixth months, and 

 perhaps for a longer period. 



The mucous membrane of the stomach is smooth up to the 

 end of the second month ; during the third month it becomes 

 much folded, especially at the pyloric end, and in the course of 

 the fourth month the glands commence to develop. In the 

 intestine the villi appear towards the end of the second month, 

 and the glands of Lieberkiihn about the beginning of the fourth 

 month. The large intestine is at first closely similar to the 

 small intestine, and contains numerous villi, which about the 

 fourth or fifth month become united by folds of the mucous mem- 

 brane to form a honeycomb pattern. Peyer's patches appear 

 about the sixth month. 



2. The Pharynx. 



The pharynx requires special notice on account of the im- 

 portance of the structures developed in connection with it. 



From the first the pharynx is distinguished from the rest of 

 the length of the alimentary canal by its great width. 



