272 THE EVOLUTION OF MAN. 



intestinal canal, as well as the mesentery, the thin, fibrous 

 membrane by which the intestinal canal is connected with 

 the ventral side of the vertebral column. 



The history of the fourth secondary germ-layer, or the 

 intestinal-glandular layer, is very simple and clear. Its 

 only product is the intestinal cellular covering, or the Epi- 

 thelium of the intestinal canal with all its appendages, the 

 large and small intestinal glands, among which are the 

 lungs, liver, salivary glands. (Cf. Plates lY., Y.) 



