TRANSMISSION AND CONDUCTION 217 



oral levels usually being at first more rapid than those 

 of the more apical. In most individuals, particularly 

 those of Pleurobrachia and Beroe, a second region of 

 high susceptibility is present at the extreme oral end of 

 the row, where new plates are developing as the animal 

 grows. This region is less susceptible than the apical 

 end, but more susceptible than the middle regions of 

 the row. As regards its relation to the gradient of the 

 row, this region corresponds to the secondary posterior 

 growing region of many animals (see chap. viii). Since 

 it is normally less completely under the control of the 

 apical region, as various facts indicate, this oral region 

 becomes physiologically isolated and shows independent 

 rhythm at a very early stage of the action of inhibit- 

 ing agents. Complete disappearance of transmitted 

 impulses occurs first at the apical end and progresses 

 orally along the row, except as regards the extreme oral 

 region. Even after transmission is completely abolished 

 in a particular region, independent movements of slight 

 amplitude may continue for some time in single plates. 

 Usually the apical and the extreme oral regions are dead 

 and opaque white in consequence of colloid changes before 

 inhibition in the least susceptible regions is complete. 

 This sequence of events, which occurs in essentially the 

 same way in all the rows at about the same rate, affords 

 a beautiful example of the gradual obliteration of apical 

 dominance and of the progressive physiological isolation 

 of various levels of the gradient, as determined by the 

 differences in susceptibility at different levels. 



The plate row also shows certain very interesting 

 features as regards reversal of the direction of transmis- 

 sion. Such reversal is most easily brought about in 



