CENTRALIZATION AND CEPHALIZATION 143 



The aggregation of the whole central nervous system in 

 a single ganglion or pair of ganglia, as in these forms, 

 does not of course mean that these animals have attained 

 a higher degree of integration than the vertebrates, but 

 merely that the degree of integration is higher than 

 would be the case in similar forms with a more diffuse 

 system. In the adult ascidian the function of the 

 ganglion as an integrating organ is certainly not highly 

 developed, but the ascidian tadpole represents a rather 

 high degree of centralization and the high degree of 

 morphological centralization in the adult appears to be 

 rather the consequence of the earlier course of develop- 

 ment than directly associated with conditions in the 

 adult. The degree of integration in the rhabdocoeles 

 and polyclads is unquestionably much higher than in 

 the triclad Planaria, where the central nervous system 

 includes not merely the cephalic ganglia but the longi- 

 tudinal cords and apparently some of their branches 

 (Fig. 19, p. 1 2 2) . The degree of integration corresponding 

 to a particular degree of centralization or cephalization 

 in a particular group depends upon the hereditary poten- 

 tialities of the protoplasm concerned as regards the 

 development of nervous structure and function. 



CENTRALIZATION IN RELATION TO AXIATION 



According to current views, the nervous system is 

 primitively diffuse and uncentralized and centralization 

 appears only in the course of evolution. I have endeav- 

 ored to show that the localization and differentiation of 

 nervous structure, and the development of nervous func- 

 tion constitute only a part of a developmental process, 

 that they must depend upon physiological conditions 



