Development of the Digestive Canal of the American Alligator | Page 5

i��, which may be traced through twenty-five or thirty sections in this series. The epithelium of the stomach is fairly thick, and consists of five or six layers of compact, indistinctly outlined cells with spherical nuclei. Ventrad to the stomach is seen, in figure 5F, a section of the duodenum, i, which extends, with gradually diminishing caliber, for twenty-five or thirty sections caudad to the posterior limit of the stomach, where it opens to the yolk-sac and is lost.
The section that cut this embryo in the posterior region of the stomach also passed through the hindgut in the region of the posterior appendages, figure 5G. There the intestine, i, is a distinct, cylindrical tube which extends, with not much variation in caliber, and with little variation in position, from this point to the cloaca. Followed cephalad, towards the posterior intestinal portal, it gradually diminishes in caliber, as did the foregut on approaching the anterior intestinal portal. The epithelium consists here of three or four layers of compactly arranged cells, and has about the same appearance as in the oesophagus and duodenum.
Figure 5H represents a section through the cloacal region, cl, showing the openings into the cloaca of the Wolffian ducts, wdo. Just anterior to these openings the cloaca opens ventrally into a small, anteriorly-projecting pouch, the rudiment of the allantois.
Caudad to the openings of the Wolffian ducts the cloaca extends ventrad as a narrow, solid tongue of epithelium towards the exterior, figure 5I, and fuses with the superficial ectoderm at the caudal end of a prominent ridge that lies in the mid-ventral line between the posterior appendages. In this embryo the cloaca has no actual opening to the exterior; the walls of the part that projects towards the exterior are in close contact, except in the region of the openings of the Wolffian ducts, as is shown in figure 5H.
Owing to the coiling of the end of the long tail the plane of the section, as is seen in figure 5I, passes through the posterior end of the embryo no less than four times. In the most posterior of these four sections of the tail, beginning slightly caudad to the section here shown, is seen a small cavity which may be called the post-anal gut, pag. It has thick walls, and extends for about thirty-five sections in the series under discussion. Its lumen is very large in its caudal region, figure 5I, pag, and tapers gradually cephalad until it disappears. Posteriorly the post-anal gut ends quite abruptly not very far from the extreme tip of the tail.
Figure 5J is a composite drawing from reconstructions of the enterons of two embryos of approximately this stage. One of these reconstructions was plotted on paper from a series of transverse sections; the other was made in wax from a series of sagittal sections. For the sake of simplicity the gill clefts are not represented, and the pharynx, mouth, and liver are represented in outline only. For the same reason the lung rudiment of one side only is shown.
The relative size of the pharynx, ph, as seen in the figure, is smaller than it is in reality because of the small dorso-ventral diameter (the only one here shown) compared to the lateral diameter. The end of the lung rudiment, lu, is slightly enlarged and lies in a plane nearer to the observer than that of the oesophagus, oe, though this is not well shown in the figure.
The oesophagus, oe, diminishes slightly in caliber for a short distance caudad to the origin of the lungs, then gradually increases in caliber until it suddenly bends to the side (towards the observer) and merges into the wide stomach, i��. The stomach, which is irregularly conical in shape, lies in a place slightly nearer the observer than the end of the lung rudiment mentioned above.
Lying to one side of the stomach and duodenum, and extending cephalad beyond the end of the lung rudiment is the liver, li, whose outline is only roughly shown here by the broken line. The stomach opens rather abruptly into the duodenum, d, which slopes back towards the plane of the oesophagus (away from the observer).
The projection from the side of the duodenum, pan, not well figured here, indicates the position of the pancreas, better shown in the next reconstruction. The duodenum extends only a short distance caudad to this point and then opens, aip, to the yolk-sac.
The yolk-stalk, or unclosed region of the enteron, is still of considerable extent, though its exact boundaries are not easy to determine. The distance between the anterior and posterior intestinal portals is approximately shown in the figure under discussion.
The hindgut is cylindrical in cross section and of about the same diameter throughout, except for a slight enlargement in the cloacal region.
The post-anal gut is not shown here;