G36 



THE STOMOD.EUM. 



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



TIL e 



Fi<;. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR END 



OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS. 



<'l>. epiblast; Sp.c. spinal canal ; ch. notochord; it. p. neurenteric caual ; hi/, hypo- 

 blast; p.a.g. postanal gut; pr. remains of primitive streak folded in on the ventral 

 side; al. allantois ; me. splanchnic mespblast ; OH. point where anus will be formed; 

 p.c. perivisceral cavity ; am. amnion; so. somatopleure ; sp. 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 posterior part first becomes reduced to a small 

 rudiment 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 part which at an earlier period opened 

 into the cloaca. 



In Teleostei the vesicle at the end of the tail, discovered by Kupffcr, 

 (tig. 34, hyp] is probably the equivalent of the vesicle at the end of the 

 postanal gut in Elasmobranchii. 



In Petromyzon and in Amphibia there is a well-developed postaual 

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

 shewn in the embryo of Bombiiiator in fig. 420. 



Amongst the amniotic Vertebrata the postanal gut is less developed 

 than in the Ichthyopsida. A neurenteric canal is present for a short 

 period in various Birds (Gasser, etc.) and in the Lizard, but disappears 

 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 morphological significance of the postanal gut and of the neuren- 

 teric canal has already been spoken of in Chapter xn. } p. 267. 



The Stomodceum. 



The anterior section of the permanent alimentary tract is formed 

 Ly an invagination of epiblast, constituting a more or less consider- 

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

 extremity of the alimentary tract. 



