PHYSIOLOGICAL GRADIENTS. 157 



the degree of inhibition, acclimation or recovery in growth, de- 

 velopment, motor activity, etc., in different body-regions or in- 

 dividuals. 



The data on susceptibility have demonstrated the existence in 

 a large number of organisms of definite susceptibility gradients 

 as characteristic and as the earliest distinguisable features of 

 axiate pattern, not only for the organism as a whole, but for 

 various axiate organs and parts. In all cases the differences in 

 susceptibility correspond to differences in structure, rate of 

 growth, development and differentiation, and in certain cases it 

 has been possible to show that susceptibility gradients correspond 

 to gradients in oxygen consumption and carbon dioxide pro- 

 duction. 



In various cases it has been possible to follow the apico-basal 

 or antero-posterior gradient from the egg and to show the modi- 

 fications which occur during the course of development, and the 

 appearance of new gradients in particular organs, or in connec- 

 tion with agamic reproductive processes. In the simpler organ- 

 isms the susceptibility gradients are wholly or largely superficial. 

 In the ciliate infusoria, for example, axiation and morphological 

 differentiation are, at least to a very large extent, confined to 

 the ectoplasm, and in the forms which have been examined the 

 ectoplasm alone shows a gradient in susceptibility (Child, 5 I4&). 

 As regards many plant cells the same is also true. In many of 

 the monosiphonous algae with elongated cells the apico-basal sus- 

 ceptibility gradient is very distinct within the limits of a single 

 cell (Child, 'i6c } e, '170., b, } 2oa} as well as in the axis as a whole. 

 Modifications with advancing age, cessation of growth, budding, 

 etc., appear very clearly in the changes in susceptibility. 



In some animals the primary gradients persist throughout 

 life, while in others the original gradients may completely dis- 

 appear during development. In the development of the hydroid, 

 for example, a very distinct gradient appears in the unfertilized 

 egg and cleavage stages, and the planula shows a well marked 

 apico-basal gradient. But as the time of attachment of the plan- 

 ula approaches, this gradient gradually becomes less and less dis- 

 tinct, and the planula attaches itself by the end which was orig- 

 inally apical, a fact which has long been known. After attach- 



