HISTOGENESIS— CLASSIFICATIOX 71 



vital properties as the cell. The consensus of opinion is, however, 

 that the intercellular substance is derived originally from the cell, 

 is replenished by the cell, is dependent upon the cell for its nourish- 

 ment, and is incapable of activity or existence apart from the cell. 



A scientific classification of the tissues is at present impossible. 



The foregoing list of tissue derivations shows how unsatisfactory 

 is any attempt at classification on the basis of histogenesis, many 

 tissues which are morphologically similar being derived from two 

 or even all three of the blastodermic layers. 



The following is the usual classification of adult tissues: (i) Epithe- 

 lial tissues; (2) connective tissues; (3) blood; (4) muscle tissue; (5) 

 nerve tissue. 



Of these, epithelium and connective tissue may be regarded as the 

 more elementar}' tissues, being common to both plants and animals. 

 Blood is sometimes classified among the connective tissues. Muscle 

 and nerve are the most highly specialized tissues and are peculiar 

 to animals. While the individuahty of the tissues classified is 

 recognized, the physiological necessities of nutrition, innervation, etc., 

 scarcely permit the existence of any one tissue alone by itself. Thus 

 blood and blood vessels permeate almost all tissues except epithehum, 

 while the latter is evervwhere traversed bv elements of nerve tissue. 



