i 7 8 



THE ENTODERMAL CANAL AND ITS DERIVATIVES 



two to four rows of nuclei. In 20 mm. embryos, vacuoles appearing in the epithelium give the 

 esophagus the appearance of having several lumina. The result of vacuole formation is to increase 

 the size of the lumen. In later stages the wall of the esophagus is folded and ciliated epithelial 

 cells appear in 44 mm. embryos. The number of cell layers in the epithelium increases until at 

 birth they number nine or ten. Glands are developed as ingrowths from the epithelium. The 

 circular muscle layer is indicated at 10 mm. by a circular layer of myoblasts, but the longitudinal 

 muscle layer does not form a definite layer until 55 mm. (F. T. Lewis in Keibel and Mall, vol. 2). 



Stomach. The stomach appears in embryos of 4 to 5 mm. as a laterally 

 flattened fusiform enlargement of the fore-gut caudal to the lung anlages (Fig, 

 171). Its epithelium is early thicker than that of the esophagus and is sur- 

 rounded by a thick layer of 

 splanchnic mesoderm. It is 

 attached dorsally to the body 

 wall by its mesentery, the 

 greater omenlum, and ventrally 

 to the liver by the lesser omen- 

 turn. The dorsal border of the 

 stomach enlarges to form the 

 fundus and greater curvature. 

 The dorsal wall grows more 

 rapidly than the ventral wall 

 and thus produces the convex 

 greater curvature. The whole 

 stomach becomes curved and 

 its caudal end is carried 

 ventrad and to the right 

 (Fig. 162). This forms a 

 ventral concavity, the lesser 



curvature, and produces the first flexure of the duodenum. The rapid 

 growth of the gastric wall along its greater curvature also causes the stomach 

 to rotate upon its long axis until its greater curvature or primitive dorsal 

 wall lies to the left, its ventral wall, the lesser curvature, to the right (Fig. 

 162). The original right side is now dorsal, the left side ventral in position, 

 and the caudal or pyloric end of the stomach is ventral and to the right of 

 its cardiac or cephalic end. The whole organ extends obliquely across the 

 peritoneal cavity from left to right. The change in position progresses rapidly 

 and is already completed in embryos of 12 to 15 mm. (beginning of the second 

 month) . The rotation of the stomach explains the asymmetrical position of the 



Pharynx 

 Root of tongue 



Thyreoid 

 Tip of tongue 



Rathke's pocket 

 Trachea 

 Stomach 



Liver 

 Dorsal pancreas 



Hepatic diverti- 

 culum 



Yolk-stalk 



Allantois 



Mesonephric duct 



Cloaca 



Hind-gut 



FIG. 171 A. Median sagittal section of a 5 mm. embryo to 

 show the digestive canal (modified after Ingalls). 



