Into the origin of the varying arrangements of the pancreatic ducts it is not possible to enter in detail. In some cases, e.g. the Rabbit (Kölliker), the two lobes and ducts arise from a division of the primitive gland and duct. In other cases, e.g. the Bird, a second diverticulum springs from the alimentary tract. In a large number of instances the primitive condition with a single duct is retained.
Postanal section of the mesenteron. In the embryos of all the Chordata there is a section of the mesenteron placed behind the anus. This section invariably atrophies at a comparatively early period of embryonic life; but it is much better developed in the lower forms than in the higher. At its posterior extremity it is primitively continuous with the neural tube ([fig. 420]), as was first shewn by Kowalevsky.
The canal connecting the neural and alimentary canals has already been described as the neurenteric canal, and represents the remains of the blastopore.
In the Tunicata the section of the mesenteron, which in all probability corresponds to the postanal gut of the Vertebrata, is that immediately following the dilated portion which gives rise to the branchial cavity and permanent intestine. It has already been shewn that from the dorsal and lateral portions of this section of the primitive alimentary tract the notochord and muscles of the Ascidian tadpole are derived. The remaining part of its walls forms a solid cord of cells ([fig. 423], al´), which either atrophies, or, according to Kowalevsky, gives rise to blood-vessels.
Fig. 423. Transverse optical section of the tail of an embryo of Phallusia mammillata. (After Kowalevsky.)
The section is from an embryo of the same age as fig. 8 IV.
ch. notochord; n.c. neural canal; me. mesoblast; al´. hypoblast of tail.
In Amphioxus the postanal gut, though distinctly developed, is not very long, and atrophies at a comparatively early period.
In Elasmobranchii this section of the alimentary tract is very well developed, and persists for a considerable period of embryonic life. The following is a history of its development in the genus Scyllium.
Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.