1896. ] INTESTINAL TRACT OF BIRDS. 139 
After it leaves the duodenum, the dorsal mesentery expands into 
a great, almost circular, fold, with the middle mesenteric vein 
running out to the yolk-sac in the centre of the fold. The gut is 
suspended at the circumference of this circular fold, and, m the 
simple type, is thrown into a number of corrugated folds around 
the circumference, which closely resemble the corrugated folds 
in the Alligator. At the posterior part of this circumferential 
part of the gut is the point where the ceca are attached, and the 
ceca run forward along the sides of the posterior part of this 
loop. Ina simple case such as in this young bird the edge of the 
mesentery corresponding to its line of attachment, and represented 
by a dotted line in the figure, passes directly into the edge of the 
mesentery of the rectum. But in most fully grown birds the part 
of the gut with the attached cx#ca has been rotated under the 
rectum, that is to say over it as seen in the diagram, until the 
point of attachment of the cxca is brought close up to the 
starting-point of the duodenum. Consequently, when the gut is 
lying on the table with its primitive ventral side uppermost, the 
rectum and the rectal vessel are covered along the greater part of 
their length by the circular part of the gut. Finally, individual 
folds, from among the numerous small corrugated folds of the 
circular loop of gut, increase enormously in length, and Dr. Gadow 
has shown that the number of the loops that grow out, and the 
mode in which they lie, folded over or under each other within the 
body, are characteristic of avian groups. Where the folded loops 
come in contact with each other, minor short circuitings take 
place in the veins, and it occasionally happens, notably with 
Parrots, that secondary sheets of connective tissue, usually con- 
taining masses of fat, bind loops belonging to different parts of 
the circular fold very closely together. But even in these cases, 
and without difficulty in most birds, these loops may be dissected 
from each other, and the primitive circular loop of mesentery 
becomes apparent and is seen to contain the median branch of the 
mesenteric vein. The series of figures in this communication 
exhibit the gut when this unfolding dissection has been per- 
formed. 
The rectum, or last part of the gut, in the vast majority of 
cases retains its primitive straight position, and is closely attached 
to the dorsal wall of the body-cavity by the posterior part of the 
primitive straight mesentery. The rectal vessel or posterior 
mesenteric vessel runs in this. It leaves the common stem of the 
portal vein very close to the anterior mesentery or duodenal vessel, 
and runs backward tothe cloaca. Just in front of the cloacaa large 
median vessel leaves this and runs upward to the surface of the 
kidneys. There it forks, and each fork, after receiving several 
veins from the parietes, runs forward along the under surface of 
the kidney. 
I shall now proceed to describe the deviations from this ground- 
type so far as I have had the opportunity of following them in the 
main groups of birds. The kaleidoscopic variety, in which the same 
