METABOLIC GRADIENTS 53 



striking in character that the desirability of comparative 

 study of different forms at once became evident. Up 

 to the present time some fifty species of animals from 

 various groups have been examined by means of the 

 susceptibility method, either in the adult or embryonic 

 stages or in both, in the attempt to determine to what 

 extent regional differences or gradients in metabolic 

 condition with respect to the axial or any other directions 

 in the body are characteristic features of the animal 

 organism.' 



In each form examined a more or less distinct and 

 regular gradient in susceptibility has been observed in 

 the direction of the major axis of the body and in many 

 cases gradients in the direction of the minor axes and of 

 the axes of various organs and parts as well.^ 



^ The forma examined include twelve species of ciliate infusoria 

 among the protozoa, the post-embryonic or adult stages of the fresh- 

 water hydra, and three species of hydroids among coelentrates; one 

 ctenophore, eleven species of turbellaria, and certain larval stages of one 

 trematode among the flatworms. Dr. L. H. Hyman, workmg under my 

 direction, has examined in the same way nine species of oligochete anne- 

 lids and one polychete. Susceptibility studies have been made upon the 

 eggs and embryonic or larval stages of the following forms: starfish, 

 sea-urchin, the polychete annelids Nereis, Chaetopterus, Arenicola, 

 Hydroides among the invertebrates, and two species of fishes and the 

 salamander and frog among the vertebrates. 



2 The data concerning susceptibility gradients, so far as they have 

 been pubhshed, will be found in the following papers: Child, "Studies 

 on the Dynamics of Morphogenesis and Inheritance in Experimental 

 Reproduction, I-V, VII, VHI," Jour, of Exper. ZooL, X, XI, XIII, XIV, 

 XVI, XVII, 1911-14; "Studies, etc., VI," Archh fur Entwickelirngs- 

 mechanik, XXXVII, 1913; "Certain Dynamic Factors in Experimental 

 Reproduction and Their Significance for the Problems of Reproduction 

 and Development," Archiv fiir Entwickelungsmechanik, XXXV, 1913; 

 "Susceptibility Gradients in Animals," Science, XXXIX, No. 993, 1914; 

 "The Axial Gradient in Ciliate Infusoria," Biol. Bull., XXVI, 1914; 

 "Axial Gradients in the Early Development of the Starfish," Amer. 

 Jour, of Physiol., XXXVII, 1915. 



