Development of the Digestive Canal of the American Alligator

by A. M. Reese

The allantois, al, extends cephalad for some distance from the floor of the cloaca. Some distance caudad to the cloaca, near the end of the much coiled tail, is seen the post-anal gut, pag. This structure as has been noted above, is quite distinct from the other parts of the enteron. It is of elongated, pyriform outline, with the pointed end extending cephalad.

In the narrow space between the stomach and the duodenum is the elongated pancreas, pan, opening by two or more short ducts into the duodenum.

The liver, li, in the figure under discussion, has about twice the area of the stomach. It extends caudad and dorsal about the same distance as the latter organ, but it extends ventrad and cephalad far beyond the boundaries of the stomach.

Extending along the ventral border of the liver is a long narrow duct, apparently the bile duct, bd. It connects, caudally, with the anterior end of the pancreas, while at its other extremity, near the antero-ventral corner of the liver; it ends blindly.

The transverse sections now to be described have been selected from the series from which the reconstruction, just described, was made.

[Figure 6B] represents a typical section through the pharynx. Its plane is approximately shown by the line 400 of [figure 6A] though the plane apparently does not cut the eye, e. The pharynx, ph, has here the outline of an irregular V. Its walls, except at the outer angles of the clefts, g¹, are composed of but a single layer of cells. In the dorsal wall these cells are flattened, while in the ventral wall they are more rounded. This difference in the shape of the cells accounts for the slightly greater thickness of the floor over that of the roof of the pharynx. The gill clefts no longer communicate with the exterior.

[Figure 6C] represents the caudal half of the embryo in the plane 475 of figure 6A. The section of the pharynx, ph, is here crescentic in outline, and the pharyngeal walls, especially the floor, are somewhat thicker than in the more anterior section just described. Lying a short distance dorsad to the pharynx are seen two small, thick-walled openings, ty; these are the rudiments of the thymus glands. They are here quite distinct from the enteron, and may be traced through a large number of sections, being in some regions solid and of a smaller diameter than in the present section.

[Figure 6D] is in the region of the line 500 in figure 6A. The thymus rudiments, ty, have about the same appearance as in the preceding figure, except that they are somewhat larger. The pharynx, ph, is much smaller than in the last section, and though somewhat crescentic in outline, its convex side is dorsal instead of ventral in position. The pharyngeal walls are here thicker, and consist of two or three layers of cells, instead of the single layer of more anterior sections.

In the median plane the floor of the pharynx is pushed down, as a solid tongue of cells, gs, the anterior edge of the glottis. Ventrad and laterad to the glottis a crescentic condensation of mesoblast represents the beginning of the laryngeal cartilages, la.

Two or three sections caudad to the one just described, the two layers of which the tongue of cells from the floor of the pharynx is composed separate slightly at the bottom to form a small cavity, the trachea, ta; this condition is shown in [figure 6E], which represents part of a section through the plane 532 of figure 6A.