Development of the Digestive Canal of the American Alligator

by A. M. Reese

Eighty-five sections ([figure 7], X) caudad to the one under discussion the trachea divides into the two bronchi. These bronchi gradually separate from each other until, at the point at which they open into the lungs, about eighty sections caudad to their point of separation, they lie on either side of the ventral third of the oesophagus.

[Figure 7B] represents a section through the plane 480 of figure 7. The section is just cephalad to the heart, and passes through the caudal third of the lungs, lu, which have the same appearance as in the preceding figure; also through the extreme cephalic end of the liver, li. The lungs here much more nearly fill the body cavity than in the preceding figure. The section being caudad to their openings into the lungs the bronchi do not, of course, show.

The oesophagus, oe, is here of much less diameter than in the preceding figure, but is still laterally compressed. Its wall is somewhat thicker than in the more cephalic region, the increase being mainly due to the greater thickness of the connective tissue layer, though the epithelium is also slightly thicker because of an increase in the length of the lining cells. Instead of lying almost entirely ventrad to the lungs, as in the preceding figure, the oesophagus here lies directly between them.

[Figure 7C] represents a section through the plane 627 of figure 7. The plane of the section passes through the opening of the stomach, i´, into the duodenum, d. The cross section of the stomach is somewhat larger than that of the oesophagus, but it differs from the more anterior region mainly in the character of its walls. These are much thicker than in the oesophagus; in the mesoblast which forms the greater part of their thickness, muscle fibers are beginning to differentiate. The epithelial layer also is thicker than in the oesophagus; it consists of tall columnar cells that, at places, are thrown into small folds, [figure 7I]. These folds, even under the low magnification used, are more evident than is shown in the present figure. The pylorus, py, is wide and, as has been noted in connection with figure 7, is situated far cephalad to the caudal end of the stomach. It opens into the side rather than into the end of the duodenum, which projects cephalad as a short blind pouch, d. The stomach and duodenum, in this section, are almost completely surrounded by the liver, li.

[Figure 7D] represents a section through the plane 680 of figure 7.

The stomach, i´, which is cut through its middle region, is somewhat larger than in the preceding figures, though its walls have about the same character. Its outer walls are continuous, to a considerable extent, with the tissue of the surrounding body wall, especially in the region just caudad to the plane of the present section.

The duodenum, being cut through a double loop (see [figure 7]), is seen in two places, dorsally where it is cut through the edge of one loop, and ventrally where it is cut square across. In both sections the structure is the same, as might be expected, [figure 7J]. The surrounding mesoblast is differentiated into muscle fibers, figure 7J, ml, which form a fairly distinct layer; inside of this layer is a tall columnar epithelium, ep´, which is thrown into prominent folds. A thin layer of mesoblast, probably the submucosa, sl, lies beneath the epithelium and projects up into the folds. About ten or twelve folds are seen in any one section; only the larger ones are well seen in [figure 7D].

[Figure 7E] shows a section through the plane 770 of figure 7. It is in the region of the umbilicus, u, and the extreme caudal end of the stomach which has been called the gizzard, gz. The small size of the gizzard is due to its being cut near its caudal margin. The enteron is here cut in no less than seven places: the reason for this will be evident on examination of the plane of the section as shown in [figure 7]. Dorsal to the gizzard the section cuts the so-called caecum, ce, a little nearer its anterior end than is shown in figure 7. The duodenum, d, is cut at five points, and has about the same structure as in the preceding figure. The character of the duodenal loops that causes the rather curious appearance of the present figure will be readily understood by reference to figure 7, though the reconstruction is not mathematically accurate. The ventral projection of the lower loops of the duodenum into the umbilicus is seen both in the present figure and in the reconstruction. The loop of the duodenum that, in the sections, is seen to lie directly ventrad to the gizzard, in the reconstruction is shown too much to the side of the latter organ. The descending loops of the duodenum are cut in such a way that the surrounding mesoblast forms a continuous mass of tissue.

[Figure 7F] represents a section through the plane 901 of figure 7. The section passes through the kidneys, k, the edge of one posterior appendage, pa, the large intestine, il, and two regions of the small intestine, i.

The large intestine is here a thick walled, cylindrical structure, il, hanging from a thin mesentery, ms, in the much reduced body cavity. The layers of its wall are much more fully differentiated than in the more anterior regions of the enteron. The epithelium is here stratified instead of simple columnar, and the folds into which it is thrown are broader and less numerous than in the duodenum above described.