THE BODY-CAVITIES 343 
mesentery. The mesogastrium becomes modified by the dis- 
placement of the stomach, the outgrowth of the duodenal loop, 
the formation of the omentum, and by the development of the 
pancreas and spleen in it. (See below.) 
The second division of the mesentery is related to the longest 
division of the intestine, but as this arises from a relatively very 
small part of the embryonic intestine, its dorsal attachment. is 
short and the roots of the mesenteric arteries are grouped 
together. The third division is relatively long and not very 
deep; at its base it approaches near to the mesogastrium, to 
which it is attached by the root of the intermediate division. 
The Origin of the Omentum (mainly ajter Broman). In a 
preceding section we saw that the accessory mesentery is con- 
tinued back on the right side (at the stage of seventy-two hours) 
by a fold of the dorsal mesentery of the stomach known as the 
plica mesogastrica (ig. 120). The stomach is already displaced 
somewhat to the left, hence the dorsal mesentery is bent also, 
and the plica mesogastrica arises from the angle of the bend 
(Fig. 120). The ventral mesentery of the stomach, including 
the meatus venosus and liver, remains in the middle line. Thus 
the body-cavity on the right of the stomach is divided into two 
main divisions, viz., the general peritoneal cavity lateral to the 
plica mesogastrica and liver, and another cavity between the 
plica mesogastrica and liver on the one hand, and the stomach 
on the other; the latter cavity has two divisions, a dorsal one 
between the plica mesogastrica and upper half of the stomach 
(recessus mesenterico-entericus) and a ventral one between the 
liver (meatus venosus) and stomach (recessus hepatico-entericus), 
which are continued anteriorly into the pneumato-enteric recesses. 
Subsequently, they become entirely shut off from the peritoneal 
cavity, but at present (stage of Fig. 120) they communicate 
with it by a long fissure bounded by the accessory mesentery in 
front, by the plica mesogastrica above, and the meatus venosus 
below; this opening may be called the hiatus communis recessum ; 
it corresponds to the foramen of Winslow of mammals (cf. Fig. 
193 A). 
As development proceeds, a progressive fusion of the right 
dorsal border of the liver with the plica mesogastrica takes place 
in a cranio-caudal direction, thus lessening the extent of the 
hiatus. 
