58 INDIVIDUALITY IN ORGANISMS 



of experimentation along this line have demonstrated 

 beyond a doubt the existence of such gradients as a 

 general feature of the constitution of the animal body. 



Such susceptibility gradients may be demonstrated, 

 not only by the course of death over the body, but by 

 the different degrees of retardation or inhibition of 

 growth and development at different levels under the 

 same experimental conditions. I have described such 

 retardation or inhibition gradients as observed in the 

 flatworm Planaria,^ and in the development of the sea- 

 urchin I have found it possible to alter and control to 

 a high degree the form and proportions of the larva 

 through the differences in susceptibility along the axes 

 to various reagents. Such gradients are also very 

 clearly evident in many cases described by various 

 authors of the effect of external conditions of various 

 kinds of development. The abnormal forms produced 

 in such experiments almost invariably indicate the exist- 

 ence of axial differences in susceptibility. The gradient 

 which appears in such cases is usually the acclima- 

 tion gradient, the regions of highest metabolic rate 

 being least susceptible and so least affected, but if 

 the external factor acts with sufficient intensity or if 

 acclimation does not occur, the differences in suscepti- 

 bility are parallel with the metabolic gradient itself. In 

 the embryo of the frog, which has been much used for 

 experiments of this sort, various experimental conditions 

 may retard or inhibit developmental processes in the 



^ Child, "Studies on the Dynamics of Morphogenesis and Inherit- 

 ance in Experimental Reproduction, IV, Certain Dynamic Factors in 

 the Regulatory Morphogenesis of Planaria dorotocephala in Relation 

 to the Axial Gradient," Jour, of Ex per. ZooL, XIII, 1912. 



