120 FOUNDATIONS OF BIOLOGY 



into secondary specialized tissues (muscular tissue, nerve 

 tissue, etc.) for one function or another the simple life 

 processes of Hydra are adequately provided for by the 

 specialization of isolated cells or small cell groups within 

 ectoderm and endoderm. (Fig. 65.) 



The bodies of all animals above the Coelenterates are built 

 up of three primary layers, which, as development of the 



Fia. 65. Hydra. Transverse section, highly magnified. Outer layer of cells, 

 ectoderm; inner layer, endoderm. Between, mesogloea, represented by a line. (After 

 Shipley and McBride.) 



individual proceeds, give rise to the secondary tissues and 

 thereby form a relatively complex body. This third primary 

 layer, known as the mesoderm, typically is developed, as we 

 have described earlier, from the endoderm and comes to 

 occupy the position held by the mesogloea of Hydra; that is, 

 between the ectoderm and the endoderm. 



The development of the mesoderm is the key to the ad- 

 vance in body organization of higher animals, because it 

 makes possible a radical change in plan that involves the 

 establishment of a body cavity, or COELOM, in which are dis- 



