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 
Argus giganteus ; 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, and it 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. Asa 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. I 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 ex- 
panded into a branching system of folds. This occurs in birds 
belonging to widely different groups, and must be taken as a con- 
yergent resemblance. 
