THE PHARYNX AND MESENTERON. 403 



dorsal line ; while, on the floor of the buccal cavity, the boundary 

 line lies in front of the root of the tongue ; the whole of the tongue 

 being formed from the mesenteron, and being therefore covered 

 with hypoblastic epithelium. 



The palate is formed, about the fifteenth day, by a pair of 

 horizontal ridges which grow inwards from the sides of the 

 buccal cavity, and, meeting each other in the median plane, fuse 

 to form a horizontal shelf (Fig. 151, pl), which separates the 

 nasal chamber above from the buccal cavity below. The fusion 

 ■of the two halves of the palate proceeds from before backwards ; 

 and the palate ends with a free posterior edge, behind which 

 the nasal and buccal chambers are continuous with each other 

 (Fig. 151, tp). 



4. The (Esophagus. 



The hinder end of the pharynx narrows very rapidly, and 

 passes abruptly into the straight tubular oesophagus (Figs. 150, 

 and 151, to). It has not yet been determined whether the 

 oesophagus of the rabbit, like that of the chick and tadpole, 

 passes through a stage in which it is solid for a time. 



5. The Stomach and Intestine. 



The stomach becomes evident, as a distinct dilatation of the 

 alimentary canal, about the thirteenth day; its long axis at first 

 corresponds with that of the body, but later on it shifts its 

 position, and becomes placed at first obliquely, and then almost 

 directly across the body. 



The intestine undergoes changes corresponding fairly closely 

 with those already described for the chick. The lengthening 

 of the intestine is effected almost entirely in two situations, 

 giving rise to two ventrally directed loops. Of these, the 

 proximal or duodenal loop is formed immediately behind the 

 stomach, and in the rabbit attains a considerable length (Fig. 

 160, e). The distal or vitelline loop is formed by elongation of 

 the > -shaped loop of the intestine already present on the 

 twelfth day, and from the apex of which the yolk-stalk, yk, 

 ; arises ; the vitelline loop attains an enormous length in the 

 I rabbit. 



A short length of the intestine, between the duodenal and 

 i. "vitelline loops, remains in the rabbit, as in the chick, stationary 



D D 2 



I 



