PHYSIOLOGICAL GRADIENTS. l6l 



finally, differential susceptibility, as a relation dependent pri- 

 marily upon quantitative rather than upon specific or qualitative 

 differences in the physiological state or activity of protoplasm, 

 provides a simple and adequate basis for the interpretation of 

 much of the work on experimental teratogeny and many of the 

 teratological forms occurring as " accidents " in nature. The 

 cases of cyclopia and microcephaly in fishes experimentally pro- 

 duced by Stockard ('07, '09, '10, '11, etc.) are essentially similar 

 to the inhibited types of head in Planaria (Child, 'iia, 'i$c, pp. 

 105-117) and to the differential inhibitions in the sea urchin 

 (Child, 5 i6rf) and in amphibia (Bellamy, '19, and further data 

 not yet published). All these cases involve a greater degree of 

 inhibition of apical or anterior and median as compared with 

 basal, posterior and lateral regions and all are non-specific in 

 origin, i.e., can be produced by the action of various agents and 

 conditions. 



The susceptibility method makes no pretense of being an exact 

 quantitative method of measuring metabolism or oxidation, nor 

 is it to be regarded as taking the place of any other method of 

 investigating physiological condition or rate of metabolism or 

 oxidation. Its chief value is as a supplement to other methods. 

 In the first place it enables us to demonstrate the existence of 

 certain characteristic, non-specific, regional differences in physio- 

 logical condition in organisms, which because of their unicellular 

 character or their small size, or because of the complications in- 

 troduced by separating different body regions are not available 

 material for other more direct and more exact methods. Even 

 in th/ese forms, however, the action of external chemical agents 

 is in general from the surface inward, consequently the informa- 

 tion given by the susceptibility method concerns first of all the 

 superficial regions of the cell or body, but it is possible in many 

 cases to learn something concerning differential susceptibility of 

 internal parts and organs. 



Second, by the modification and control of development 

 through differential susceptibility, the method enables us to show 

 that the differences in condition indicated by differences in sus- 

 ceptibility are fundamental factors in organismic pattern. The 

 conclusions concerning the relation between susceptibility and 



