THE ALIMENTARY CANAL. 283 



(iii) The proctodeum is a barely perceptible pitting-in of 

 the epiblast, at the hinder end of the alimentary tract, which 

 forms the anal or cloacal aperture. 



Up to the end of the fourth day the alimentary canal is nearly 

 straight ; but from this time it grows more rapidly than the part 

 of the body in which it lies, and soon becomes markedly convo- 

 luted ; it retains its connection with the mid-dorsal wall of the 

 body cavity by means of the mesentery. 



The several regions of the alimentary canal, and the various 

 organs formed in connection with it, will now be taken in order ; 

 and the more important points in their developmental history 

 described. 



2. The Pharynx. 



Almost from the first, there is a great difference between the 

 anterior or pharyngeal portion of the mesenteron, which is shallow 

 dorso-ventrally but very wide from side to side (Fig. 118, tp) ; and 

 the hinder part, from the oesophagus to the cloaca, which is narrow 

 and cylindrical. 



Towards the end of the second day, pouch-like folds of hypo- 

 blast grow out in pairs from the sides of the pharynx, towards 

 the surface. These correspond exactly, in their relations and their 

 mode of formation, to the gill-pouches of the tadpole ; and like 

 these, they grow outwards until they meet the external epiblast, 

 with which they fuse. At a slightly later stage, the fused patches 

 of epiblast and hypoblast become perforated to form the gill-clefts, 

 which place the gill pouches, and therefore the pharynx, in direct 

 communication with the exterior. 



Of these gill-pouches, four are formed on each side of the 

 neck, and are developed in order from before backwards. The 

 most anterior one is the hyomandibular gill-pouch (Fig. 124, 

 hm) ; and the succeeding three are the first, second, and third 

 branchial pouches respectively. 



The parts of the side walls of the pharynx between the suc- 

 cessive gill-pouches are spoken of as the visceral arches ; their 

 boundaries are indicated on the surface of the neck by grooves, 

 marking the lines along which the hypoblastic walls of the gill- 

 pouches meet and fuse with the external epiblast, as shown on 

 the right-hand side of Fig. 124. The first or most anterior of 

 these visceral arches is the mandibular arch (Figs. 124 and 125, 



