636 THE STOMODuEUM. 



point a little behind the anus, where the postanal section of the canal 



Fi(}. 425. Diagrammatic longitudinal section through the posterior end 

 OF an embryo Bird at the time of the formation of the Allantois. 



ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. neurenteric canal ; hy. hypo- 

 blast; p.a.fi. postanal gut; j^''- lemains of primitive streak folded in on the ventral 

 side; al. allantois; me. si^lanchnic mesoblast ; a?i. point where anus will be formed; 

 p.c. perivisceral cavity; am. amnion; so. somatopleure; sj). splanchnopleure. 



was thinnest in the previous stage, it becomes solid, and a rupture here 

 occurs in it at a slightly later period. 



The atrophy of this part of the alimentary tract having once commenced 

 proceeds rapidly. Tlie })osteri()r part first becomes reduced to a small 

 ludiment near the end of the tail. There is no longer a terminal vesicle, 

 nor a neurenteric canal. The portion of the postanal section of the 

 alimentary tract, just behind the cloaca, is for a short time represented 

 by a small rudiment of the dilated 2)art which at an earlier period opened 

 into the cloaca. 



In Teleostei tlie vesicle at the end of the tail, discovered by Kupffer, 

 (fig. 34, h'l/v) is ])robably the equivalent of the vesicle at the end of the 

 postanal gut in Elasmobranchii. 



In Petromyzon and in Ampliibia there is a well-developed postanal 

 gut connected with a neurenteric canal which gradually atrophies. It is 

 shewn in the embryo of Bombinator in fig. 420. 



Amongst the amniotic Vertebrata the postanal gut is less developed 

 than in tlie Ichthyopsida. A neurenteric canal is ])resent for a short 

 period in various Birds (Gasser, etc.) and in the Lizard, but disappeai'S 

 very early. There is however, as has been pointed out by Kolliker, a well 

 marked postanal gut continued as a narrow tube from behind the cloaca 

 into the tail both in the Bird (fig. 425, p.a.g.) and Mammals (the Rabbit), 

 but especially in the latter. It atrophies early as in lower forms. 



The moi'phological significance of the ])ostanal gut and of the neuren- 

 teric canal has already been .spoken of in Chapter xii., p. 267. 



TJie Stoniodceum. 



The anterior section of the permanent alimentary tract is formed 

 by an invagination of epiblast, oustituting a more or less consider- 

 able pit, with its inner wall in contact with the blind anterior 

 extremity of the alimentary tract. 



