THE EAE AND THE ALDIENTAKY CANAL. 145 



papillae, probably gustatory in function, which are lost at the 

 time of the transformation to the frog. 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



1 . General Account. 



The alimentary canal of the frog, like that of other Verte- 

 brates, is developed in three lengths : (i) the mesenteron (Fig. 

 69, T), which is formed, as already described, by a process of 

 splitting amongst the yolk-cells, and which corresponds to the 

 mesenteron or gastrula cavity of Amphioxus : the mesenteron 

 of the frog gives rise to almost the whole length of the ali- 

 mentary canal, from the pharynx to the rectum ; and from it 

 are developed the gill-clefts, the thyroid, the thymus, the lungs, 

 the liver, the pancreas, and the bladder, (ii) The stomatodaeum 

 (Fig. 69, DS) is a pitting in at the anterior end of the body, 

 from which the mouth opening and buccal cavity are formed, 

 and in connection with which the lips and teeth are developed, 

 (iii) The proctodseum (Fig. 60, PD) is a pocket-like depression 

 at the hinder end of the body, which gives rise to the anal or 

 cloacal opening. 



The mesenteron. The mode of development of the mesen- 

 teron, up to the stage shown in Fig. 55, has already been 

 described. At its first appearance, and throughout the early 

 stages, the mesenteron has walls of very unequal thickness ; the 

 roof or dorsal wall (Fig. 56) being thin ; and the floor or ventral 

 wall being of great thickness, owing to the large size of the yolk- 

 cells which form it. 



After separation of the mesoblast cells as a distinct layer, 

 and the definite formation of the notochord, this difference 

 becomes still more marked, the roof of the mesenteron (Fig. 

 56, T) consisting of a single layer of hypoblast cells, while the 

 floor is formed by the thick mass of yolk-cells ; at the sides the 

 transition from the thin roof to the thick floor is a somewhat 

 abrupt one. 



As the central nervous system is formed, and the shape of 

 the embryo becomes more clearly established, the mesenteron 

 acquires more definite characters (cf. Figs. 55, T; 60, MN). By 

 enlargement of its anterior end a wide pharyngeal cavity 



