DIGESTIVE ORGANS 



221 



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 proctodexmi, similar to the 

 stomodeum, and opening later into the 

 mesenteron in the same way. In the adult 

 it is impossible to find any Unes separating 

 the three regions, stomodeum, mesenteron 

 and protodeum. 



The protodeum lies wholly behind the entrance 

 of the urogenital ducts into the cloaca. The 

 ectoderm of the stomoderm 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 en toderm passing between them. The posterior 

 of these, Seessel's pocket, is of unknown signifi- 

 cance. The other, Rathke's pocket (fig. 282), lies 

 just in front of the oral plate. It marks the point 

 of invagination of the hypophysis (p. 157) and 

 remains open for a time as the hypophysial duct 



(fig- 159)- 



In some teleosts, where the stomodeal ingrowth 

 is slight, the mouth appears at first as a pair of 

 perforations in the oral plate, these later coalesc- 

 ing to form the permanent mouth. This condition 

 lends plausibility to the view that the vertebrate 

 mouth has afisen from the coalescence of a pair 

 of gill clefts. 



Except in the higher mammals the ento- 

 dermal part of the aUmentary canal contains 



a large amount of food yolk in the early stages. p^^ 232.-Reconstruc- 

 In the sauropsida this is so abundant that the tion of alimentary canal of 

 whole cannot be contained in the body walls, annantoTs\?a°i;;f/l doSaj 

 and hence it causes the ventral side of the f.' glo"is; h, hyoid arch; /», 



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

 Canal to protrude as a yolk-sac (fig. 329), dibular and maxillarv arches; 



which is gradually absorbed with the diges- JiS'v'.",;;°i,°±thT., 



tion and removal of the yolk by the blood- visceral arches; vi, vitelline 

 , stalk; w, WolflBan body. 



vessels. 



The first differentiation in the mesenteron is the development of a 

 ventral diverticulum, the anlage of the liver, which arises just caudal 



