288 



THE ABDOMEN AND PELVIS 



so acquires a dorsal mesentery. A loop of gut is thus formed, 

 and as the abdominal walls do not grow at the same rapid rate, 

 it descends through the large patent umbilicus and lies in the 

 exo-ccelom within the umbilical cord. At the end of the third 

 month the abdominal cavity is large enough to contain all its 

 viscera, and the intestinal loop returns into the abdomen. The 

 exo-ccelom then becomes completely obliterated, and no trace 



CEsophagus 



Rudiment of respiratory system 

 Notochord j 



Medulla spinalis 



Ectoderm of embryo 



Liver diverticulum branching in septum transversum 

 j Stomach 



Dorsal pancreas rudiment 



Ventral pancreas rudiment 

 Peritoneal part of ccelum 

 Caecum 



Peritoneal part of 

 celum 



Wolffian duct 



Rectum 



Hypophysis- 



Cloacal membrane 



Rathke's pouch 



Ectoderm of amnion 



Mesoderm of amnion 



Cerebral hemisphere 



Umbilical cord 

 Placental mesoderm 



Pericardium | 

 Gall-bladder rudiment 



'I Yolk-sac 



Septum transversum 



FIG. 90. Schema of a Sagittal Section through a Human Embryo, 

 after the rotation of the intestinal loop. 



of it should be found in the umbilical cord at birth. Thus 

 during the second and third months the human embryo normally 

 possesses an umbilical hernia. If the loop of gut fails to return 

 within the abdomen, or if the exo-ccelom remains patent and 

 retains its connection with the peritoneal cavity, a congenital 

 umbilical hernia will result. 



Congenital diaphragmatic hernia is due to imperfect develop- 

 ment of the septum transversum, which ought to separate 

 the pleural and peritoneal sacs completely from one another. 



