138 Mil, p. OHALMBES MITOIIBLL 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 gigantevs ; intestinal tract, from a chick after 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 pan- 

 creas, 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, audit contains the anterior mesenteric vein. The duo- 

 denum, 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. Por 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 pre.sent. The bridging-vessels from the duodenum are short 

 circuitings which have been perpetuated. 



The duodenum, usually a simple loop, is in some instances ex- 

 panded into a branching system of folds. This occurs in birds 

 belonging to widely different groups, and must be taken as a con- 

 vergent resemblance, 



