THE STOMACH AND INTESTINE. 291 



The rectum, or terminal part of the intestine, grows very 

 slowly, and remains nearly straight throughout the whole period 

 of development. The boundary between the small intestine and 

 the rectum is marked by the two rectal diverticula, which appear 

 as a pair of small pouch-like outgrowths (Fig. 123, gh) about 

 the end of the fifth day ; these grow rapidly, and by the eighth 

 or ninth day have attained a considerable length. The rectum 

 itself remains short ; in the later days of incubation it dilates 

 very greatly, and shortly before the time of hatching the bursa 

 Fabricii arises as a dorsal outgrowth from its distal end. 



The mesentery. The alimentary canal, along its whole length, 

 is at first closely attached to the dorsal wall of the body cavity, 

 immediately below the notochord. 



The pharynx, or most anterior division of the alimentary 

 canal, retains these relations throughout life. The oesophagus 

 shifts ventralwards to a slight extent, owing to the intrusion of 

 mesoblast between it and the notochord. Further back the 

 ventral shifting is much more marked ; and the whole intestinal 

 region, from the stomach to the rectum, becomes situated some 

 distance ventral to the notochord, remaining, however, connected 

 with the dorsal wall of the body cavity by a vertical, laterally 

 compressed sheet of mesoblast, the mesentery. 



An exception to this statement must be made with regard to 

 the short portion of the intestine between the duodenal and 

 vitelline loops, which, as already noticed, remains in close con- 

 nection with the dorsal body wall throughout life. 



As the duodenal and vitelline loops of the intestine lengthen, 

 the mesentery grows, keeping pace with them, and becoming 

 still further reduced in thickness ; it ultimately forms a thin sheet, 

 consisting of two epithelial layers, derived from the peritoneum, 

 and inclosing between them a very thin layer of mesoblast, along 

 which the blood-vessels run to and from the alimentary canal. 



The terminal part of the alimentary canal, or rectum, like 

 the anterior part, remains closely connected with the dorsal body 

 wall throughout life, the mesentery in this region only attaining 

 a comparatively slight development. 



6. The Proctodeum. 



The proctodeum is a slight depression of the skin at the 

 hinder end of the body, beneath the tail (Fig. 123). It develops 



U 2 



