400 TILE BABBIT. 



respectively, which meet and open into the mesenteron, and 

 so place it in communication with the exterior. 



The alimentary canal is at first straight, or merely follows 

 the curvature of the body, and is situated immediately ventral 

 to the notochord. It remains in this condition, throughout life, 

 in the pharyngeal and oesophageal regions, and also at its ex- 

 treme hinder end ; but along the rest of its extent it shifts 

 ventral wards, remaining connected with the dorsal wall of the 

 body cavity by a mesentery (Fig. 165, mh). In the region of 

 the small intestine the alimentary canal increases in length far 

 more rapidly than the body of the embryo, and becomes in 

 consequence thrown into folds, in order that it may be accom- 

 modated within the body cavity. 



2. The Stomatodseum. 



The relations of the stoniatodaeal pit are practically the same 

 as in the chick. Perforation of the stoniatodaeal membrane is 

 effected at an early stage, before the end of the tenth day. The 

 pituitary body arises, still earlier, as a diverticulum from the 

 posterior and dorsal angle of the stoniatodaeal pit ; its further 

 development has already been described in the section dealing 

 with the formation of the brain (p. 376). 



3. The Buccal Cavity and Pharynx. 



The pharyngeal region of the mesenteron is, from the first, 

 distinguished by its great width (Figs. 158 and 159, tp). Early 

 on the tenth day, the branchial pouches arise as paired diverticula 

 from the sides of the pharynx ; and, opposite to the outer ends 

 of the branchial pouches, branchial grooves are formed on the 

 surface of the neck, marking out the boundaries of the several 

 visceral arches. The walls of the branchial pouches and of the 

 corresponding branchial grooves come into close contact, a thin 

 branchial membrane (Fig. 158, eb), consisting of epiblast and 

 hypoblast, without any intervening mesoblast, alone separating 

 the two. This membrane, however, remains intact ; and in the 

 rabbit none of the gill-clefts are ever completely formed, or 

 open to the exterior at any stage of development. There are 

 also no traces of gills, either external or internal, at any period 

 in the rabbit. 



The visceral arches are well developed, and on the twelfth 



