172 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



is formed an enlargement, due to a ventral bulging of the gut wall, which marks 

 the anlage of the cacum and the boundary line between the large and small in- 

 testine. The caecal anlage gives rise later both to the adult cacum and to a more 

 distal appendage, the vermiform process, which lags in development and remains 

 small. 



Succeeding changes in the intestine consist (1) in its torsion and coiling due 

 to its rapid elongation, and (2) in the differentiation of its several regions. As 



Rathke's pocket 



Hypophysis 



Thyreoid 



Notockord 



Pericardium 



AUantois 



Dorsal pancreas 

 Ventral pancreas 



Cloacal membrane 



CcECum 

 Urogenital simis I ^ ^\ — zi^)^^^^ 



I \ \ Peritoneal cavity 



Tail gut \ Mesonephric duct 



Rectum 



Fig. 179. — Diagram, in median sagittal section, showing the digestive canal of a 9 mm. human embryo 



dapted from Mall). X 9. 



the gut elongates in 9 to 10 mm. embryos, the intestinal loop rotates. As a result, 

 its caudal limb lies at the left and cranial to its cephalic limb (Fig. 179). At 

 this stage the intestinal loop enters the coelom of the umbilical cord, thus causing 

 a temporary umbihcal hernia. 



The small intestine soon lengthens rapidly and at 17 mm. (Fig. 180) forms 

 loops within the umbihcal cord. Six primary loops occur and these may be 

 recognized in the arrangement of the adult intestine (Mall, Bull. Johns Hopkins 

 Hosp., vol. 9, 1898). In embryos of 42 mm. the intestine has returned from the 



