96 THE ORIGIN OF THE NERVOUS SYSTEM 



There are of course many cases in which the gradient 

 does not determine in the egg any such gross structural 

 differentiation as the yolk gradient, but such differences 

 are incidental rather than fundamental, and depend 

 upon differences in the specific constitution of the proto- 

 plasm of different eggs, differences in nutritive conditions, 

 etc. But sooner or later the structural differences 

 appear, always in definite and orderly relations to the 

 gradients, and it has been shown that the localization 

 and relative size and proportions of parts along an axis 

 can be altered experimentally by altering the length, 

 steepness, or direction of the gradient. 1 I believe that 

 the evidence at hand warrants the statement that it has 

 been experimentally demonstrated that such a gradient 

 in a specific protoplasm constitutes an adequate basis 

 for the initiation and so for the orderly progressive com- 

 plication, with reference to the particular axis which the 

 gradient represents, of the differentiation characteristic 

 of the process of development in that particular proto- 

 plasm. Once again it may be pointed out that the gradi- 

 ent does not determine the character of differentiation 

 and the kind of organs produced at different levels in 

 different organisms. These are determined by the 

 specific inherited constitution of the protoplasm. 



Where symmetry gradients as well as polar gradients 

 are present the situation is of course further compli- 

 cated and orderly differences arise in more than one 

 direction. Similarly, as particular organs are localized 

 and develop, new organ gradients or axes may arise at 

 any angle to the original axis in consequence of local 

 conditions, until in the higher animals the axial relations 



1 Child (191 ic, 19156, pp. 142-46, igi6d), Bellamy (1919). 



