620 MR. P, CHALMERS MITCHELL ON THE [June 16, 
The mid-gut is thrown into three well-marked loops: the first of 
these is long and narrow; the second is long, is much more open, 
and shows a tendency to be thrown into a very rough spiral. In 
the chick and in two adults I found no trace of the yolk-sac 
diverticulum, but its place of attachment was marked by a distinct 
and strong remnant of the ventral mesentery ; in a third adult, as 
shown in the figure, this mesentery ran to a minute vestige of the 
yolk-sac, placed nearly at the summit of the middle loop. The 
third loop of the mid-gut is wide, and along it the ceca run in the 
fashion characteristic of birds in which these are functional; where 
the duodenum lay over this third loop, a bridging vein ran from 
the ceca to the duodenal branch of the mesenteric vein. The 
rectum is very long and is thrown into secondary folds. 
It is obvious that the gut of Opisthocomus exhibits a definite 
divergence of a simple nature from what I tried to show, in the 
paper referred to above, to be the primitive type of avian intestines. 
The chief character of the typical intestinal folds is that the mid- 
gut, from the duodenum to the insertion of the long ceca, is a 
simple loop, thrown into short folds at the circumference of an 
almost circular expansion of the mesentery, and bearing near 
its median point a vestige of the yolk-sac. Such a condition 
occurs almost unmodified in the Struthious birds, in the Gallide 
and Cracide, and, among aquintocubital birds, in Chawna and 
Palamedea, in Himantopus, Glareola, and Caprimulgus. So far 
as I have had opportunity of examining them, and I have now 
more than doubled the material wpon which I! first formed 
the conclusion, nearly every group of birds contains members . 
approaching this primitive type. The divergences consist in the 
stretching out and twisting of secondary loops of this primitive 
circular loop, while the direction of the divergences is, on the whole, 
identical in each group. Opisthocomus, inasmuch as its mid-gut 
displays differentiation into three well-marked subsidiary loops, 
has advanced beyond the Gallidew, Cracide, and Struthious birds. 
Its mode of divergence differs from that of the Tinamou, in which 
the first and third subsidiary loops are very Jong, but in which the 
region hearing the volk-sac vestige and corresponding to the median 
loop is not expanded. Neglecting the fact that Pterocles and the 
Pigeons are aquintocubital, while Opisthocomus is certainly quinto- 
cubital, the latter from the form of the gut is intermediate between 
Pterocles and the Pigeons. In these three the mid-gut has three 
loops, the central loop bearing the yolk-sac vestige: as in Pterocles 
the ceca are long; the middle loop shows a trace of the spiral 
formation which is characteristic of the higher Pigeons. Among 
quintocubital birds Opisthocomus shows the closest resemblance to 
the Cuculidz, in which also the ceca are Jong and the mid-gut is 
thrown into three loops, the median loop bearing the yolk-sac 
vestige. So far as argument may be based upon the formation of 
the mid-gut, either Huxley’s’ suggested relationship between 
* “On the Classification and Distribution of the Alectoromorphe and 
Heteromorphe,” P, Z. 8. 1868, p. 294, 
