DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 343 



ing is formed. The portion of the gut caudal to the anus, known as the caudal 

 gut, normally disappears. 



Histogenesis of the Gastrointestinal Tract. 



The wall of the primitive gut is composed of two layers the entoderm which 

 lines the lumen, and the splanchnic mesoderm which borders on the ccelom or body 

 cavity. While the germ layers are still flat, the entoderm is a single layer of flat 

 cells with bulging nuclei, but after the closure of the gut the cells become col- 

 umnar. The splanchnic mesoderm is composed of two layers the mesothe- 

 lium bordering on the ccelom, the cells of which gradually change from flat 



Mesentery 



Mesothelium 



Long. 



Trans. 



muscle 



Epithelium * 



Stroma 



FIG. 306. Transverse section of the small intestine of a pig embryo of 32 mm. Bonnet. 



to rather high, and a number of indifferent, branching mesenchymal cells 

 lying between the mesothelium and entoderm. The entoderm is destined to 

 give rise to the general epithelial lining of the gastrointestinal tract and to all 

 the glands connected with it. The mesothelium around the gut forms a part of 

 the general mesothelial lining of the ccelom, its cells apparently changing back 

 to a flat type. The mesenchymal tissue is destined to give rise to all the con- 

 nective tissue and smooth muscle of the tract. The circular layer of muscle 

 appears first, the longitudinal next, both appearing during the third and fourth 

 months, and last of all the muscularis mucosae (Fig. 306). 



THE Mucous MEMBRANE. The mucous membrane is formed by the 

 epithelium (entoderm) and the subjacent mesenchymal tissue. In its develop- 



