206 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In the majority of vertebrates the blastopore closes behind, so that 

 the anus is a new formation, although it arises 

 in the line of closure. In the amniotes this 

 opening is preceded by the formation of a 

 pocket, the proctodeum, similar to the 

 stomodeum, and opening later into the 

 mesenteron in the same way. In the adult it 

 is impossible to find any lines separating the 

 three regions, stomodeum, mesenteron and 

 proctodeum. 



The proctodeum lies wholly behind the entrance 

 of the urogenital ducts into the cloaca. The ecto- 

 derm of the stomodeum extends inward as far as the 

 posterior teeth, following the outline of the jaws. On 

 the dorsal side of the oral cavity two pits persist for 

 some time, the limits of ectoderm and entoderm pass- 

 ing between them. The posterior of these, SeessePs 

 pocket, is of unknown significance. The other, 

 Rathke's pocket (fig. 253), lies just in front of the 

 oral plate. It marks the point of invagination of the 

 hypophysis (p. 148) and remains open for a time as 

 the hypophysial duct (fig. 148). 



In some teleosts, where the stomodeal ingrowth is 

 slight, the mouth appears at first as a pair of per- 

 forations in the oral plate, these later coalescing to 

 form the permanent mouth. This condition lends 

 plausibility to the view that the vertebrate mouth has 

 arisen from the coalescence of a pair of gill clefts. 



Except in the higher mammals the ento- 

 dermal part of the alimentary canal contains a 

 large amount of food yolk in the early stages. 

 In the sauropsida this is so abundant that the 

 whole cannot be contained in the body walls, 

 and hence it causes the ventral side of the 

 canal to protrude as a yolk-sac, which is 



gl glottis; h, hyoid 'arch; u, gradually absorbed with the digestion and re- 

 liver; lu. lung; md. mx. man- , - , , ,, -, , -, 

 dibular and maxillary arches; niOVal of the yolk by the blood-VCSSels. 



n, nasal pit; 0) omphaiomesa- The first differentiation in the mesenteron 



raic vein; s, stomach; v, vis- 

 ceral arches; vi, vitelline is the development of a ventral diver ticulum, 



the anlage of the liver, which arises just caudal 

 to the head. This divides the alimentary canal into pre- and post- 



F I G. 208. Reconstruc- 

 tion of alimentary canal of 



