TIIK BODY CAVITIES, DIAPHRAGM AND MESENTERIES 



181 



extend toward the midplane cranial to the heart and communicate with 

 each other (Fig. 188). Laterally they are not continuous with the extra- 

 embryonic ccelom, for the head of the embryo separates early from the 

 underlying blastoderm. The pericardial cavities also are prolonged 

 caudally until they open into the pleuro-peritoneal cavities. These 

 in turn communicate laterally with the extra-embryonic ccelom. In an 

 embryo of 2 mm. the ccelom thus consists of a U-shaped pericardial cavity, 

 the right and left limbs of which are continued 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 limited. On each side, at the level 

 where the vitello-umbilical trunk (Fig. 

 88) courses to the heart, the splanchnic 

 mesoderm and the somatic mesoderm 



united (cf. Fig. no). Thus 



Pericardial cavity 

 Surface of fore-gut 



'Pleuro-peritoneal canal 

 Entoderm of gut 



-Peritoneal cavity 

 -Extra-embryonic ccelom 

 -Wall of yolk sac 



FIG. 1 88. Diagrammatic model of 

 the fore-gut and ccelom in an early human 

 embryo, viewed from above and behind 

 (modified after Robinson). 



are united (cf. .Fig. no). Thus is 

 formed the septum transversum, which incompletely partitions the ccelom 

 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 mesocardia (Fig. 190 A). Into the ventral mesentery, caudal to 

 the septum, grows the liver. This portion of the ventral mesentery 

 gives rise dorsally to the lesser amentum of the stomach, and, where it fails 

 to separate from the septum transversum, it forms the ligaments of the 

 liver. Ventrally it persists as the falciform ligament (Fig. 190 ). 



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 amentum of the 

 stomach, the mesoduodenum, the mesentery proper of the small intestine, 

 the mesocolon, and the mesorectum. 



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

 continuous with each other and consist of a mesothelium overlying con- 



