112 C. M. CHILD 



'12, '16 b), while in differential acclimation and recovery the 

 changes are opposite in direction. The fact that axial rela- 

 tions in these widely different organisms can be experimentally 

 altered and controlled in similar ways through the differential 

 effect of inhibiting agents indicates very clearly the funda- 

 mental identity of the physiological axes as metabolic gradients. 

 In the light of these facts, the further demonstration of the 

 existence of axial metabolic gradients in algae among plants 

 (Child, '16 a) and in protozoa, coelenterates, flatworms, echino- 

 derms, annelids, fishes, amphibia and birds among animals and 

 their correspondence with developmental gradients of other kinds 

 are sufficient to establish the general and fundamental signi- 

 ficance of such gradients in the development and differentiation 

 of organisms. 1 



The modifications of form, localization, and differentiation by 

 differential inhibition, acclimation and recovery in the sea urchin 

 larvae and in the reconstitution of pieces of Planaria afford a 

 basis for the interpretation of many other cases of experimental 

 teratogeny and of various teratological forms observed in nature. 

 The cases of cyclopia in fishes produced experimentally by Stock- 

 ard and others are simply differential inhibitions like those pro- 

 duced in the sea urchin and Planaria. Median regions are more 

 inhibited than lateral, and lateral organs approach the median 

 line or become median. Differential inhibition in the antero- 

 posterior axis in the early stages of development may produce 

 various gradations from the normal form to acephaly. In seg- 

 mented animals, where a region of high metabolic rate arises 

 secondarily at the posterior end of the primary gradient and be- 

 comes a growing region from which new segments develop, a 

 secondary gradient arises, with its region of highest rate in the 

 posterior growing region, and differential inhibition along this 

 gradient may produce certain characteristic teratological forms. 



In short, the examination of teratological forms which are 

 not the direct result of some factor acting locally, but are pro- 

 duced by agents or conditions which affect the body as a whole, 



1 Child, '12, '13 b, '14 a, '15 a, '15 c, '16 a, '16 b, '16 c, Hyman, '16. A con- 

 siderable part of these data is still unpublished. 



