54 INDIVIDUALITY IN ORGANISMS 



In organisms or parts with a radial structure gradients 

 in susceptibility may commonly appear in the direction 

 of the radial axis, and in those animals and developmental 

 stages where the outer body surface consists of active 

 living cells and is not covered by a heavy cuticle or 

 exoskeleton a susceptibility gradient from the surface 

 inward has been frequently observed. 



In the simpler multicellular animals and in those 

 unicellular organisms which possess definite permanent 

 axes, the susceptibiHty gradients along the main body 

 axes often persist from the beginning of development 

 throughout life without essential change, but in many 

 cases they undergo various changes during the course 

 of development : they may disappear and new gradients 

 arise with advancing differentiation and the appearance 

 of new organs, or they may undergo reversal in direction 

 in some or most of the tissues of the body. In all cases, 

 however, so far as observed, such changes occur in a 

 definite and orderly way, so that the relation between the 

 original and the final condition is essentially constant 

 and characteristic for a given species. In spite of the 

 developmental alterations, it is true, as far as observa- 

 tions go at present, that for each of the main axes of the 

 body a definite susceptibiHty gradient exists, at least 

 during the earlier stages following the appearance of the 

 axis, and a definite relation exists between the direction 

 of the gradient from high to low susceptibiHty along a 

 given axis and the course of development and differen- 

 tiation and the functional correlation of different parts 

 with reference to the same axis. 



The following figures will serve to show something 

 of the definiteness of the gradient along the apico-basal 



