BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



197 



man they disappear early. The development of the vaginal sacs which grow out from the 

 inguinal region of the peritoneal cavity into the scrotum will be described in Chapter VIII. 



The Omental Bursa or Lesser Peritoneal Sac. According to Broman, the 

 lesser peritoneal sac is represented in 3 mm. embryos by a peritoneal pocket which 

 extends cranially into the dorsal mesentery to the right of the esophagus. A 

 similar pocket present on the left side has disappeared in 4 mm. embryos. Lateral 

 to the opening of the primitive peritoneal sac, a lip-like fold of the mesentery is 

 continued caudally along the dorsal body wall into the mesonephric fold as the 



J?icjhtuml>i//ca/ vein 

 Ventral mesentery 



"Right lobe 

 & Liver 



Lesser 



periioneat 

 'sac 



Plica 

 Venae Cewae 



Dorsal aorta 

 Neural tuhf 



Left umbilical Vein 



Ectoderm of 

 body wall 



Left lobe 

 >f Li ver 



Ventral 

 mesentery 



Duodenum 



Dorsal 

 mesentery 



Left post . 

 Cardinal vein 



Notochord 



FIG. 192. Diagrammatic view of an embryo of 7 to 9 mm. showing the position of the lesser peri- 

 toneal sac. The cranial portion of the embryo is represented as sectioned transversely, caudal to the 

 liver, so that one looks at the caudal surface of the section and of the liver and cranially into the body 

 cavities. 



plica vena cava, in which later the inferior vena cava develops (Fig. 192). The 

 liver, it will be remembered, grows out into the ventral mesentery from the fore- 

 gut and, expanding laterally and ventrally, takes the form of a crescent. Its 

 right lobe comes into relation with the plica venae cavse and, growing rapidly 

 caudad, forms with the plica a partition between the lesser sac and the peritoneal 

 cavity. Thus the cavity of the lesser peritoneal sac is extended caudally from 

 a point opposite the bifurcation of the lungs to the level of the pyloric end of the 

 stomach. In 5 to 10 mm. embryos it is crescent-shaped in cross-section (Fig. 

 132) and is bounded mesially by the greater omentum (dorsal mesentery) and the 

 right wall of the stomach, laterally by the liver and plica venae cavae and ven- 



