■ EMBRYOGENY 



123 



and the blood which they contain, and the epithehal Hning of all cavities 

 developed in this layer. The mesodermal tissues in higher forms far 

 exceed in bulk those from the ectoderm and entoderm combined. 



It will be observed from what has been said that an epithelium may 

 be derived from any one of the three layers. The epidermis is ectodermal 

 in origin, the epithelium hning the greater part of the digestive cavity 

 is entodermal, and the epithelium lining the cavities within the mesoderm, 

 including the lining of the heart and blood vessels, is mesodermal. Skele- 

 tal parts may be formed not only from the mesoderm but sometimes from 

 the ectoderm, and even in rare cases from the entoderm. 



147. Coelom. — Any cavity formed in the mesoderm and surrounded 

 by mesothelium is known as a coelom. When it is present the sex organs 

 become developed from its wall and the excretory organs open into it. 

 It is lacking in the lower Metazoa but it is present in most of the higher 

 forms, in which it may be divided into several cavities. The layer of 

 the mesoderm outside the coelom and lying against the body wall is called 

 somatic; that inside the coelom and lying against the viscera, splanchnic. 



Since three cavities develop during the embryogenies of higher animals 

 it is well to bring them into contrast, as may be done in the table which 

 follows: 



Name of cavity 



Time 

 of appearance 



Lining of wall 



Fate 



