252 THE ORIGIN OF THE NERVOUS SYSTEM 



and downward is characteristic of the definitive paths 

 in the higher vertebrates, it is much less distinct in the 

 lower vertebrates. The condition in mammals and 

 man evidently represents the most advanced stage of an 

 evolutionary process as regards these paths. Second, 

 although our knowledge of the developmental changes 

 in the nervous system, which lead up to this definitive 

 condition, is fragmentary, certain facts at hand show 

 that, in the Amphibia at least, the condition in early 

 developmental stages is not only very different from the 

 definitive condition in that group but more or less the 

 opposite of the definitive condition in the higher ver- 

 tebrates. Since the facts as they stand appear highly 

 significant in relation to the physiological gradients 

 and to the changes which they undergo in the course 

 of development, a brief survey of certain of them is 

 essential. 



In the course of anatomical and physiological studies 

 of the development of the nervous system of salaman- 

 ders, Coghill (1909, 1913, 191 4) has found that in the 

 earliest stages at which movement of the body in 

 response to tactile stimulation is possible, a definite 

 reflex mechanism has already been developed, which 

 differs in certain ways, highly significant in the present 

 connection, from the definitive mechanisms. This 

 mechanism is discussed in a later paper by Herrick and 

 Coghill (191 5) and is indicated in Figure 70, taken 

 from that paper. Certain large sensory cells in the 

 cord (giant cells of Rohon-Beard) have given rise to 

 outgrowths regarded by the authors as dendrites (see 

 pp. 196-200), which reach both the skin and the muscles, 

 and to other outgrowths (axons), which ascend the cord 



