428 C. M. CHILD 



the similarity of the differential modifications of development in 

 different species and accounts for the fact that they represent a 

 quantitative rather than a specific aspect of the action of physical 

 and chemical agents upon protoplasms. In other words, these mod- 

 ifications result primarily from differences in rate, amount, in- 

 tensity, or degree, etc., in protoplasm, rather than from differences 

 in quality, but in so complex a physicochemical system as proto- 

 plasm qualitative differences are certainly frequently determined 

 by quantitative changes. There can be no doubt, for example, 

 that rate of fundamental metabolism in the cells which give rise 

 to a new head in Planaria is one factor in determining whether or 

 not eyes or cephaUc lobes or even cephalic ganglia shall develop. 



The significance of the physiological gradients has to do with 

 these quantitative aspects of development. They represent 

 merely quantitative physiological conditions in the protoplasm, 

 which constitute factors in the realization of the hereditary 

 potentialities of that particular protoplasm. The gradients 

 create nothing; they do not determine the specific characteris- 

 tics of particular organs; they merely determine whether a 

 particular process of realization shall occur or where it shall be 

 localized and how it shall proceed. 



The fact that essentially the same developmental modifications 

 may be produced by many different agents and conditions does not, 

 of course, mean that all such agents and conditions act in exactly 

 the same way upon protoplasm. It means merely that with 

 certain ranges of intensity or concentration non-specific factors 

 or quantitative factors in their action exist. It is these fac- 

 tors which constitute the basis of differential susceptibility. 

 Undoubtedly different agents and conditions act in different 

 ways upon the same protoplasm and the same agent or condition 

 acts in different ways on different protoplasms, even upon differ- 

 ent organs or tissues within the same individual. Notwith- 

 standing these real or apparent specificities of relation between 

 protoplasm and physicochemical agents, the non-specific as- 

 pects also exist, and in the simpler organisms and the earlier 

 stages of development they play the larger part. 



