774 THE STOMOD/EUM. 



(fig- 34> hyv) 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 postanal 

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

 shewh in the embryo of Bombinator 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 



FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR 

 END OF AN EMBRYO BlRD 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.g, postanal gut ; pr. remains of primitive streak folded in on the ventral 

 side ; al. allantois ; me. splanchnic mesoblast ; an. point where anus will be formed ; 

 p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure. 



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 xii., p. 323. 



The anterior section of the permanent alimentary tract is 

 formed by an invagination of epiblast, constituting a more or 

 less considerable pit, with its inner wall in contact with the 

 blind anterior extremity of the alimentary tract. 



In Ascidians this pit is placed on the dorsal surface (fig. 9, o), 

 and becomes the permanent oral cavity of these forms. In the 

 larva of Amphioxus it is stated to be formed unsymmetrically 



