RUDIMENT OF THE INTESTINAL CANAL. 325 



and is the intestinal-glandular layer. The outer stratum 

 consists of lighter, smaller cells, and is the intestinal fibrous- 

 layer. The cavities of the mouth and the anus are the only 

 exceptions to this, because they originate from the outer 

 skin. The inner cell-coating of the entire mouth-cavity is 

 therefore furnished, not by the intestinal glandular-layer, 

 but by the skin-sensory layer, and its muscular lower layer, 

 not by the intestinal-fibrous layer, but by the skin-fibrous 

 layer. This is equally true of the wall of the anal cavity 

 (Plate V. Fig. 15). 



If the question be asked, what relation these component 

 germ-layers of the primitive intestinal wall bear to the 

 infinitely varied tissues and organs which we afterwards 

 find in the developed intestine, the answer is extremely 

 simple. The relations of these two layers to the formation 

 and differentiation of the tissues of the intestinal canal with 

 all its parts, may be condensed into a single sentence : The 

 intestinal epithelium, that is, the inner, soft cell-stratum 

 which coats the cavities of the intestinal canal and of all its 

 appendages, and which directly accomplishes the nutritive 

 process, develops solely from the intestinal-glandular 

 layer ; on the contrary, all other tissues and organs belong- 

 ing to the intestinal canal and its appendages, proceed from 

 the intestinal-fibrous layer. From this latter, therefore, 

 originates the entire outer covering of the intestinal tube 

 and its appendages ; the fibrous connective tissue and the 

 smooth muscles which compose its fleshy skin ; the carti- 

 lages which support these, for example, the cartilage of the 

 larynx and of the trachea ; the numerous blood and lymph 

 vessels which absorb nutrition from the wall of the intestine; 

 in short, everything belonging to the intestine, with the 



