TRANSMISSION AND CONDUCTION 219 



distance, but the range gradually increases with con- 

 tinued stimulation, until they arrive at the apical end. 

 On cessation of the stimulation the reverse occurs, the 

 normal apical impulses gradually extending their range 

 as the oral impulses decrease theirs, until finally the 

 normal direction is re-established. In the earlier stages 

 of the action of inhibiting agents reversal of direction 

 often occurs in Pleurobrachia without external stimula- 

 tion, beginning at the oral end and often extending over 

 the whole row. In such cases the greater degree of 

 inhibition of the more susceptible apical regions has 

 obliterated or reversed the original gradient to such a 

 degree that transmission toward the apical end becomes 

 the normal direction. Similar reversal of physiological 

 gradients has been observed in many other cases. Some 

 degree of reversal may be produced in Beroe by these 

 methods, but usually only in the oral half of the row, 

 and I have never seen the reversal extend to the apical 

 end. In Mnemiopsis and Bolina reversal has been seen 

 only at the extreme oral end and then only in the 

 presence of inhibiting agents. Apparently the excita- 

 tion waves are not high enough or not steep enough to 

 travel up the gradient in these forms. 



In Pleurobrachia and Beroe a difference in rate of 

 transmission appears, not only with difference in direc- 

 tion, but with difference in degree of excitation of the 

 animal. In a strongly stimulated individual the rhythm 

 is much more rapid and the impulses travel more rapidly, 

 in fact, almost twice as rapidly as in a quiescent animal. 



That the ctenophore plate row represents a physio- 

 logical gradient with a rise at its growing oral end is 

 demonstrated not only by the susceptibility method 



