THE ABDOMINAL CAVITY 



287 



is obtained to prevent the upward spread into the various 

 compartments of the abdomen proper. 



Congenital Hernise. In a three weeks' human embryo, 

 the alimentary canal may be subdivided into the fore-gut, the 

 mid-gut, and the hind-gut (Fig. 89). They form a tube, which 

 is closed at its cephalic and caudal ends, but which communicates 

 with the yolk-sac through the vitello-intestinal duct. At the 

 junction of the mid- and the hind-gut, the allantois forms a 

 short blind diverticulum which grows into the body-stalk. At 

 this period the mesoderm lining the ccelom, or body-cavity, of 

 the embryo is directly continuous with the mesoderm lining 



Spinal part of neural tube 



Notochord 

 Fore 

 Hind-brain 



Mid-gut 



| Amnion cavity 



Ectoderm of amnion 



Mesoderm of amnion 



Hind-gut 



Mid-brain 



Fore-brain 

 Stomatodaeum 



Pericardium (heart not shown) 



Rudiment of liver 



Allantcic diverticulum 

 Mesoderm of yolk-sac 



Umbilical orifice Entoderm 

 FIG. 89. Schema of Sagittal Section of Embryo, after the formation 

 of the Head and Tail Folds. 



the cavity exo-ccelom in which the yolk-sac lies, and con- 

 sequently the two spaces communicate with one another. The 

 anterior or cephalic part of the ccelom is destined to form the 

 pleural and pericardial sacs, while the rest becomes the peritoneal 

 cavity. As the amnion (Fig. 89), which at first lies dorsal to 

 the embryo, grows round its ends and sides, the exo-ccelom is 

 encroached on, and the body-stalk is brought closer to the 

 yolk-sac. In this way the umbilical cord is formed, and at first 

 it contains the yolk-sac, the rapidly narrowing vitello-intestinal 

 duct, and the remains of the exo-ccelom in addition to the 

 blood-vessels of the body-stalk, the whole being surrounded by 

 the amnion (Fig. 90). 



As the abdominal part of the alimentary canal increases in 

 length, it drags the peritoneum away from the dorsal wall, and 



