THE STUDY OF ORGANS. 41 



ment is like a little bng with two other little bags 

 lying one ou each side of it. Then these two other 

 little bags grow larger and meet ; at the place where 

 they meet, their walls part, so that they become 

 united; meanwhile, they lose their connection with 

 the main cavity at the opposite side, but retain their 

 connection with one another. Thus they entirely sur- 

 round the chief bag. This last, sometimes called the 

 mesenteron, is the alimentary canal, and the hollow that 

 rings it round is the body cavity. The lining of the 

 body cavity, including the covering of the mesenteries 

 (peritoneal epithelium), ^ therefore, belongs originally 

 to the hypoblast layer; it receives the general name 

 of the ccBlomie epithelium. It is very thin, and 

 all the supporting part of the wall of the body cavity 

 belongs to the mesoblast layer. 



The growth of the body cavity may take place in 

 other ways; but the above ^ is believed to represent 

 its primitive mode of formation. Thus the alimentary 

 canal and the body cavity are both differentiated from 

 the primitive archenteron, which corresponds with the 

 enteron of the Ccelenterata. Hence it is that the 



' The name epithelium was originally given to the ciliated 

 or glandular cells lining the various internal cavities of the 

 body. The term is now applied not only to these, but also to 

 surface cells belonging to the outer body-layer. 



• When the body cavity is formed in tbis way, it is called 

 an enteroccele ; when the steps of its development are 

 shortened, so that it is first seen to appear as a slit surrounded 

 by mesoblast tissue, it is called a schizocoele, or " split- 

 cavity "; this is the case in vertebrates (see p. 113). 



