In most forms the original boundary between the epiblast of the proctodæum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication.
Fig. 429. 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. mesoblast; an. point where anus will be formed; p.c. perivisceral cavity; am. amnion; so. somatopleure; sp. splanchnopleure.
In Birds the formation of the proctodæum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa Fabricii.
The proctodæum first appears when the folding off of the tail end of the embryo commences ([fig. 429], an) and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut.
The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodæum. The actual perforation of the septum between the proctodæum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of fœtal life, and the approximation of the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces.
The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctodæum, with which the bursa Fabricii communicates.
Bibliography.
Alimentary Canal and its appendages.