328 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tions. This region is now physiologically isolated from the apical region, 

 and the region of origin of the new impulse dominates all levels of the row 

 oral to it. Later the continued toxic action of the agent may inhibit this 

 dominant region; and a second physiological isolation, with again a new 

 dominant region, may appear still farther orally in the row. In the long 

 plate rows of Mnemiopsis three successive physiological isolations, each 

 with its new dominant region, have been observed under inhibiting con- 

 ditions. 



These isolations are similar, as regards relation to a gradient pattern 

 and a dominance, to establishment of posterior zooids in planarians and 

 other forms, except that in the ctenophore isolation results from decreased 

 dominance while in the planarian under natural conditions it usually re- 

 sults from increase in length but may be brought about experimentally 

 by decrease in dominance. 



On recovery from the toxic effect a gradual functional reintegration of 

 the plate row takes place, the physiologically isolated regions being again 

 finally subordinated to apical dominance. In Pleurohrachia the extreme 

 oral end of the row may become temporarily dominant after certain de- 

 grees of toxic action and direction of transmission of impulse reverses 

 over the whole row. This is a reversal of polarity by differential inhibi- 

 tion, but with recovery the original dominance and direction of transmis- 

 sion gradually reappear, replacing the reversed transmission step by step 

 along the row. 



Physiological isolation at any level of the row may also be brought 

 about by blocking the impulse from the apical organ by low temperature 

 or mechanical pressure or by direct stimulation which initiates impulses 

 independent of the apical impulse. By continued mechanical stimulation 

 at a particular level of the row a new dominant region can be established. 

 Impulses initiated in this region are transmitted orally, that is, down the 

 gradient, over the whole length of the row oral to the level of stimulation, 

 but acropetally only a short distance at first; with continued stimulation 

 acropetal transmission extends farther and may even reach the apical end 

 of the row, obliterating the apical impulse. After cessation of the stimula- 

 tion the dominant region established by it gradually disappears, and the 

 whole row is again integrated under apical dominance. 



The vertebrate heart resembles the ctenophore plate row in many re- 

 spects as regards dominance and physiological isolation. In the fully de- 

 veloped functional heart the sinus region is the pacemaker, that is, the 

 dominant region, and it develops from the high end of the primary heart 



