i8o 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



Pericardial cavity 

 Surface of fore-gut 



caudally into the paired pleuro-peritoneal cavities; these extend out into the extra- 

 embryonic ccelom. 



When the head fold and fore-gut of the embryo are developed, the layers of 

 splanchnic mesoderm containing the heart tubes are folded together ventral to 

 the fore-gut and form the ventral mesentery between the gut and the ventral body 

 wall (Fig. 190). Owing to the position of the yolk sac, the caudal extent of the 

 ventral mesentery is hmited. At the level of each side, where the vitello-umbiHcal 

 trunk (Fig. 88) courses to the heart, the splanchnic mesoderm and the somatic 

 mesoderm are united (cf. Fig. 110). Thus is formed the septum transversum, 



which incompletely partitions the coelom 

 into a cranial and caudal portion (Fig. 

 189). Cranial to the septum, the heart 

 is suspended in the ventral mesentery 

 which forms the dorsal and ventral meso- 

 cardia (Fig. 190 A). Into the ventral 

 mesentery caudal to the septum grows 

 the Hver. This portion of the ventral 

 mesentery gives rise dorsally to the lesser 

 omentum of the stomach, and, where it 

 fails to separate from the septum trans- 

 versum, it forms the ligaments of the 

 liver. Ventrally it persists as the falciform 

 ligament (Fig. 190 B). 

 Dorsal to the gut, the splanchnic mesoderm of each side is folded together in 

 the median sagittal plane to constitute the dorsal mesentery which extends to 

 the caudal end of the digestive canal (Figs. 189 and 190 C). This suspends the 

 stomach and intestine from the dorsal body wall and is divided into the dorsal 

 mesogastrium or greater omentum of the stomach, the mesoduodenum, the mesen- 

 tery proper of the small intestine, the mesocolon, and the mesorcdiim. 



The covering layers of the viscera, of the mesenteries, and of the body wall 

 are continuous with each other and consist of a mesothehum overljdng connective 

 tissue. The parietal lining is derived from the somatic layer of mesoderm and 

 the visceral covering from the splanchnic layer. 



The primitive ccelom lies in the horizontal plane, as in Fig. 188. Coincident 

 with the caudal regression of the septum transversum, the pericardial ca\'ity is 

 bent ventrad and enlarged (Fig. 191). The ventral mesocardium attaching the 

 heart to the ventral body wall disappears and the right and left Kmbs of the 



Pleuro-peritoneal canal 

 Entoderm of gut 



-Peritoneal cavity 

 -Extra-embryonic 

 -Wail of yolk sac 

 Fig. 188. — Diagrammatic model of the 

 fore-gut and crelom in an early human em- 

 bryo, viewed from above and behind (modi- 

 fied after Robinson). 



