



. 

 PREFACE 



Concerning the embryonic development and the 

 evolution of nervous structure a large body of fact and 

 hypothesis is at hand, but the problem of the origin of the 

 nervous system as an organ of excitation, transmission, 

 and integration has received much less attention. Irri- 

 tability, that is, excitability, is commonly regarded as a 

 fundamental property of living protoplasm, and it is 

 often asserted that the nervous system does not repre- 

 sent a new function, added at a certain stage of develop- 

 ment, or of evolution, to the original functional complex 

 of the organism, but rather a development of the primi- 

 tive mechanisms of excitation and transmission. Not 

 infrequently, however, particularly within recent years, 

 it has been maintained that the primary factors in 

 physiological correlation and integration consist of 

 chemical substances and their transportation by one 

 means or another. These two conceptions are difficult 

 to reconcile, for the one implies that excitation and 

 transmission are primary factors in physiological corre- 

 lation, the other that they are of secondary origin. As 

 a matter of fact, we know that excitation and transmis- 

 sion occur in protoplasm in the absence of anything which 

 we can identify as nervous structure; moreover, it is 

 difficult to conceive a living organism incapable of at 

 least some degree of excitation and transmission. 



Considered from a physiological viewpoint, the origin 

 of the nervous system must be sought in conditions 

 present before the appearance of definitely recognizable 





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