DYNAMICS — MORPHOGENESIS AND INHERITANCE 429 



And finally, the fact must be emphasized that differential, 

 inhibitions, acclimations, and recoveries in development are de- 

 pendent primarily upon the differences in susceptibility in the 

 earlier developmental stages, rather than upon those observed 

 in the fully developed animal. In Planaria, for example, as in 

 other forms thus far examined, the data indicate very clearly 

 that in the earlier stages of head-development the median region 

 is more susceptible to higher and more able to acclimate to 

 lower concentrations and intensities than lateral regions. In 

 the fully developed normal head, however, the special sensory 

 regions, e.g., the cephalic lobe and the preganglionic region are 

 the most susceptible portions of the external surface (Child, '13 b). 

 Moreover, in the fully developed animal differences in the rela- 

 tive susceptibility appear with different agents (Child, '20 b, 

 pp. 157-159). These real or apparent specificities are, how- 

 ever, of secondary origin and are associated with the progress 

 of differentiation. Their existence does not in any way con- 

 flict with, or make impossible the existence of the general quanti- 

 tative relations which determine the susceptibility gradients in 

 the less highly specialized protoplasms. 



INHERITANCE AND EXPERIMENTAL REPRODUCTION IN PLANARIA 



It is sufficiently obvious that at least certain aspects of the 

 problem of inheritance can be approached only through develop- 

 ment; that is, knowledge of the hereditary potentialities of a 

 particular protoplasm can be obtained only through the realiza- 

 tion of these potentialities in development. Moreover, it is 

 now generally recognized that so-called normal development 

 represents only a certain range of realization among the heredi- 

 tary potentialities. In other words, normal development is 

 merely the particular complex and sequence of changes which 

 is possible in a particular protoplasm under what we call normal 

 conditions. The relatively high degree of uniformity in normal 

 development results from the fact that it represents a process 

 which is, so to speak, standardized by the relatively high 

 degree of uniformity of the conditions under which it occurs. 



