CENTRALIZATION AND CEPHALIZATION 151 



body gradients. The evolutionary progress of centraliza- 

 tion and cephalization from the primitive more or less 

 diffuse localization of nervous structure in relation to 

 the general body surface is one aspect of the evolution 

 of axiate pattern. 



At present, however, we can do little more than 

 note the relation between the physiological gradients 

 and nervous localization. How it comes about physio- 

 logically that the higher levels of these gradients give 

 rise primarily and predominantly to nervous structure 

 is a problem in the physiology of localization and 

 differentiation like that pertaining to any other organ. 

 We can see that the origin of the chief aggregation of 

 nervous structure from that region of the embryo which 

 is primarily most active, most sensitive, and physio- 

 logically dominant represents a continuous sequence of 

 physiological processes, but our present knowledge 

 of living protoplasm does not permit us to analyze this 

 sequence into its physicochemical terms. 



THE POSITION OF PERIPHERAL RECEPTORS 



The apparent migration of the cell bodies of periph- 

 eral receptors from various regions of the body surface 

 toward or into the central nervous aggregation consti- 

 tutes another aspect of the evolutionary process of cen- 

 tralization. Primitively these cells differentiated in the 

 general epidermis (Fig. 18) and gave rise to nerve fibers 

 which grew toward the central nervous system and were 

 afferent in function, but in the vertebrates most of 

 these cells arise in close relation to the central nervous 

 system and send fibers peripherally as well as centrally. 

 The olfactory epithelium alone retains the epidermal 



