have been separated off the remainder of the
coelom becomes the peritoneum. At first the stomach and intestine form a
straight tube, which is connected to the dorsum of the embryo by a
_dorsal mesentery_ and to the mid-ventral wall in front of the umbilicus
by a _ventral mesentery_. Into the ventral mesentery the liver grows as
diverticula from the duodenum, so that some of the mesentery remains as
the _falciform ligament_ of the liver and some as the lesser omentum.
Into the dorsal mesentery the pancreas grows, also as diverticula, from
the duodenum, while the spleen is developed from the mesoderm contained
in the same fold. As the stomach turns over so that its left side
becomes ventral, the dorsal mesentery attached to it becomes pulled out,
in such a way that part of it forms the great omentum and part the
gastro-splenic omentum. After the caecum is formed as a diverticulum
from the intestine it is situated close to the liver and gradually
travels down into the right iliac fossa. This passage to the right is
accompanied by a throwing over of the duodenal loop to the right, so
that the right side of its mesentery becomes pressed against the dorsal
wall of the abdomen and obliterated. This accounts for the fact that the
pancreas and duodenum are only covered by peritoneum on their anterior
surfaces in man. The formation of the lesser sac is due to the turning
over of the stomach to the right, with the result that a cave, known
sometimes as the _bursa omentalis_, is formed behind it. Originally, of
course, the whole colon had a _dorsal mesocolon_ continuous with the
mesentery, but in the region of the ascending and descending colon this
usually disappears and these parts of the gut are uncovered by
peritoneum posteriorly. The transverse mesocolon persists and at first
is quite free from the great omentum, but later, in man, the two
structures fuse[1] and the fourth layer of the great omentum becomes
continuous with the posterior layer of the transverse mesocolon.

For further details see Quain's _Anatomy_ (London, 1908).

_Comparative Anatomy._--In the Amphioxus the coelom is developed in the
embryo as a series of bilateral pouches, called _enterocoeles_, from the
sides of the alimentary canal; these are therefore entodermal in their
origin, as in Sagitta and the Echinodermata among the invertebrates. In
the adult the development of the atrium causes a considerable reduction
of the coelom, represented by two dorsal c