38 THE ORIGIN OF THE NERVOUS SYSTEM 



in lower concentrations or intensities permit at least 

 some degree of acclimation or recovery. Moreover, 

 modifications of the same general type with respect to a 

 particular axial gradient can be produced in widely 

 different organisms, e.g., flatworms, echinoderms, fishes, 

 frogs. In many respects these definite developmental 

 modifications constitute the strongest evidence for the 

 relation between susceptibility and the rate of funda- 

 mental activities of living protoplasm. 1 And finally, 

 differential susceptibility, as a relation dependent 

 primarily 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 

 produced by Stockard (1907, 1909, 1910, 1911, etc.), 

 are essentially similar to the inhibited types of head in 

 Planaria (Child, 191 ia, 191 5^, pp. 105-17) and to the 

 differential inhibitions in the sea urchin (Child, 1916 d) 

 and in Amphibia (Bellamy, 1919, and further data not 

 yet published) . All these cases involve a greater degree 

 of inhibition of apical or anterior and median as com- 

 pared with basal, posterior, and lateral regions and all 

 are nonspecific 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 



1 Data on the control and modification of development through 

 differential susceptibility have appeared as follows: Child (19114, 

 1916^, 1917^); Bellamy (1919), and further data on Hydrozoa, echino- 

 derms and Amphibia are still unpublished. 



