In Teleostei the vesicle at the end of the tail, discovered by Kupffer, ([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 shewn in the embryo of Bombinator in [fig. 420].
Fig. 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. hypoblast; 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.
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 Kölliker, 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 neurenteric canal has already been spoken of in Chapter XII., p. 323.
The Stomodæum.
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 (vide p. [5]), but further observations on its development are required.