DYNAMICS — MORPHOGENESIS AND INHERITANCE 413 



new head is out of place and develops at the expense of whatever 

 regions of the body adjoin the cut surface. In fact, it may be 

 said to develop in spite of other parts of the piece, and the 

 more intense the stimulation of these parts, the less rapid and 

 complete is the dedifferentiation of the cells at the cut surface 

 and therefore the greater the degree of inhibition of head- 

 development. 



Under all conditions thus far analyzed, whether these are 

 internal, such as region of body, degree of stimulation, nutritive 

 condition, and physiological age, or external physicochemical 

 conditions, the series of head-forms from normal to acephalic is 

 essentially the same. In other words, there is no evidence to 

 indicate any specific relation between particular head-forms and 

 particular conditions or agents. Slight minor differences with 

 different agents may indicate specificities, but it is perfectly 

 evident that the series of head-forms in general represents quan- 

 titative gradations rather than specific differences in develop- 

 mental factors. 



DIFFERENTIAL iSUSCEPTIBILITY 



It has been demonstrated for several hundred species of 

 organisms, including the chief animal groups as well as many 

 plants, that from the earliest embryonic stages on, definite and 

 characteristic differences in susceptibility are relateed in a defi- 

 nite way to the physiological axes (Child, '20 b, pp. 154-162, 

 and references there given). Moreover, it has been found 

 that in general among the simpler organisms and in the earlier 

 stages of development susceptibility to a certain range of rela- 

 tively high concentrations or intensities of at least many external 

 agents varies directly, though not necessarily proportionally, 

 with the rate of certain fundamental physiological activities and 

 that the ability to become acclimated to certain lower ranges of 

 concentration and to recover from temporary exposure varies 

 in the same way. This relation between susceptibility and 

 physiological condition is fundamentally quantitative in charac- 

 ter. In the more highly differentiated stages and organisms 

 this simple relation is altered and complicated by various, ap- 



