212 THE ORIGIN OF THE NERVOUS SYSTEM 



such reversal means merely that a region of excitation 

 arises in consequence of the stimulation and that the 

 impulses initiated in it give rise to a temporary local 

 gradient which overcompensates and reverses the 

 general gradient for the time being and within a certain 

 distance from the stimulated region, and so makes 

 possible transmission in a direction opposite to the 

 normal direction. The ctenophore plate row, discussed 

 in the following section, affords beautiful examples of 

 transmission and its reversal in relation to physiological 

 gradients. 



From the present viewpoint these cases of so-called 

 neuroid transmission on ciliated surfaces appear, so far 

 as the facts are at hand, to be merely cases of general 

 cellular or protoplasmic transmission in relation to a 

 general physiological gradient or to a temporary excita- 

 tion gradient. The suggestion advanced by Parker 

 (1905, p. 419) that neuroid transmission occurs irrevers- 

 ibly in one direction and that mechanical transmission 

 is probably concerned in reversal is quite unneces- 

 sary. The directive factor appears beyond a doubt to 

 be the physiological gradient and reversal to be asso- 

 ciated with local or more or less extended reversal of 

 this gradient by local excitation, or in some cases by 

 developmental changes. 



TRANSMISSION IN THE ROW OF SWIMMING PLATES OF 

 THE CTENOPHORE 



The body of the ctenophore possesses eight rows of 

 motor organs, extending from a region near the aboral 

 or apical end toward the oral end (Fig. 67). Each of 

 the plates in a row is a flat, somewhat paddle-like 



