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Show 138 MR. P. CHALMERS MITCHELL ON THE [Jan. 14, section I made of an Alligator, is shown such a simple mode of increase in length. In fig. 2, which is drawn from the embryo of an Argus Pheasant about thirty days old, a primitive type of the avian intestine is shown, and it is easy to compare with this the simpler Alligator type and the more specialized arrangement in other birds. The avian intestine consists of three divisions, each typically supplied with a tributary of the portal vein. The first loop or duodenum Fig. 2. Argus giganteus; intestinal tract, from a chick alter incubation for thirty days. is considerably elongated, and may be folded or even spirally twisted at the free end. It contains the greater part of the pancreas, although in some cases the pancreas encroaches upon other parts of the intestine. Its mesentery is simply the elongated anterior portion of the common dorsal mesentery seen in the Alligator, and it contains the anterior mesenteric vein. The duodenum, as Dr. Gadow has shown, lies most ventrally of all the folds of the intestine, it being folded backward and downward upon the other loops. As a result of this position it frequently happens that branches of the anterior mesenteric vein leave the mesentery, and, bridging the intervening space, supply part of the posterior region of the gut. 1 have found these bridging-vessels remarkably constant in the groups in which they occur, and they seem to present a striking instance of a feature which, apparently, could only have arisen from the " accident" of contiguous position, and is fixed as a normal part of the structure. For where the part of the gut obtains its veins from this extrinsic source, the normal vein, a branch of the middle mesenteric vein which runs backward, is present. The bridging-vessels from the duodenum are short circuitings which have been perpetuated. The duodenum, usually a simple loop, is in some instances expanded into a branching system of folds. This occurs in birds belonging to widely different groups, and must be taken as a convergent resemblance. |
