THE BODY-CAVITIES 343 



mesentery. The mesogastriiim 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 after 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 (Fig. 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. 



