DIFFERENTIATION OF NERVOUS STRUCTURE 113 



tion of nervous structure is in general localized, first, 

 with reference to the physiological surface-interior rela- 

 tions, and, second, with reference to the axiate relations 

 as represented by the primary or chief body gradients. 

 The earliest appearance and the chief aggregation of 

 nervous structure is localized at or near the high ends of 

 the gradients present in the organism concerned, and 

 the general direction of nervous differentiation and of 

 nerve outgrowth during the earlier stages of develop- 

 ment is down the chief body gradients. Later of course 

 the situation becomes always more and more compli- 

 cated by the localization of new regions of high activity 

 and new gradients, which may determine locally the 

 direction of nervous differentiation and growth, but 

 the sequence of events is orderly and definite. 



THE EARLY DIFFERENTIATION OF NERVOUS 

 STRUCTURE 



The very early differentiation of nervous structure is 

 another striking feature of ontogeny. In the coelen- 

 terates nervous differentiation is so slight that it is 

 difficult to determine when it begins, but in other animals 

 we find that the central nervous system becomes recog- 

 nizable at least as a region of localized growth at the 

 anterior end soon after the establishment of the primary 

 germ layers and before other definitive organs make 

 their appearance (Figs. 13-17). 



These various facts appear to me to indicate a very 

 close ontogenetic relation between the primary physio- 

 logical gradients and nervous differentiation, and they 

 afford a physiological basis for various features of 

 nervous development which have heretofore remained 



