172 



THE ENTODERMAL CANAL AND THE BODY CAVITIES 



the left nerve supplying the ventral wall of the stomach, originally the 

 left wall, while the right vagus supplies the dorsal wall, originally the 

 right. 



In 17 mm. embryos the stomach has reached its permanent position, the cardia 

 having descended through about ten segments, the pylorus through six or seven. 



Tongue 



Laryngo-tracheal 

 groove 



Rathke's. 

 pouch 



Metane phros 



Mesonephric duct 



Allantois 



FIG. 178. Reconstruction of a 5 mm. human embryo, showing the entodermal canal and its 

 derivatives (His in Kollmann). X 25. 



Gastric pits are indicated in 16 mm. embryos, and, at 100 mm. (C R), glands cells of 

 the gastric glands are differentiated from the gastric epithelium. The gastric pits num- 

 ber 270,000 at birth but increase by fission to nearly 7 million in the adult. The cardiac 

 glands are developed early (91 mm. (C R) fetuses). 



At 10 mm. the stomach wall is composed of three layers: the entodermal epithelium, 

 a thick mesenchymal layer, and the peritoneal mesothelium. At 16 mm. the circular 

 muscle layer is indicated by condensed mesenchyma; a heavier ring forms the pyloric 

 sphincter. At 91 mm. (C R) the cardiac region shows a few longitudinal muscle fibers, 

 which become distinct in the pyloric region at 240 mm. (CR). 



