72 PATTERNS AND PROBLEMS OF DEVELOPMENT 



the same in direction with respect to developmental pattern, physiological 

 axes, or radii for certain ranges of concentration or intensity of many dif- 

 ferent agents which act on living protoplasms in different ways. 



Differential susceptibility to external agents appears in various aspects: 

 differential death, differential inhibition, differential tolerance, differen- 

 tial acclimation, differential recovery, differential acceleration, and, in 

 some cases in later stages, differential irritability and other functional dif- 

 ferentials. 



Using death or the cellular changes — cytolysis, coagulation, disintegra- 

 tion, etc. --associated with, or indicating, the approach of death, as a cri- 

 terion, we find that death gradients are characteristic features of develop- 

 mental pattern. For example, when the whole organism is equally ex- 

 posed to action of an external agent in concentration or intensity above 

 the limit or tolerance, but only slowly lethal, death occurs first in a certain 

 region of a physiological axis and progresses along the axis from that re- 

 gion. In the earlier stages of buds and of organs which develop as budlike 

 outgrowths from localized fields death progresses radially, though not al- 

 ways equally in all directions, from a region of highest susceptibility, 

 which may be central or otherwise situated in the field. As such an axis 

 develops, the radial susceptibility gradient becomes a longitudinal gradi- 

 ent. The differences in time of death at different levels of an axis may dif- 

 fer widely with different agents and with concentration or intensity of a 

 single agent; but with certain ranges of concentration or intensity of many 

 different agents, both chemical and physical, the progress of death is in 

 the same direction. 



Differential inhibition of embryonic or reconstitutional development or 

 of other activities may occur with concentrations or intensities of many 

 agents which are more or less toxic or inhibitory but not directly lethal. 

 In consequence of differential inhibition developmental form and propor- 

 tion can be modified in definite and controllable directions. 



With a certain range of lower concentration or intensity, which, of 

 course, must be determined experimentally for each species and each 

 agent, and usually for different developmental stages and for physiologi- 

 cally young and old individuals, evidences of differential tolerance and of 

 ability to acquire increased tolerance have been found in cases examined. 

 In earlier publications this differential in degree or limit of tolerance or 

 threshold of toxic or lethal action of the agent and the differential acquire- 

 ment of increased tolerance have been grouped together and called "dif- 

 ferential acclimation." Now, however, it seems desirable to distinguish, at 



