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 
