io6 THE ORIGIN OF THE NERVOUS SYSTEM 



factor in the series of events which constitute morpho- 

 logical and physiological development. From the first 

 moment when qualitative differences arise at different 

 levels of the gradients, chemical or transportation corre- 

 lation becomes possible and from this time on it becomes 

 increasingly complex and important as a factor in 

 determining the course of events. The earlier trans- 

 missive relations between certain parts may later be 

 supplemented or perhaps even more or less completely 

 replaced in some cases by transportative relations. 

 Whether or not the gradient persists as such throughout 

 the life of the individual is a question of detail to be 

 determined in each particular case. As a matter of 

 fact, in the simpler organisms it does often persist, at 

 least in the superficial regions and body wall, but it 

 does not follow from its presence in these parts that 

 the internal organs will show the same relations. The 

 changes which occur between the comparatively simple 

 physiological gradient or gradients in the egg and the 

 situation in the adult must be determined, like any 

 other aspect of development, by investigation in each 

 particular case. In these changes the hereditary mech- 

 anisms of the different protoplasms are concerned. 

 Even if the gradients in two different protoplasms were 

 exactly similar in slope and quantitative relation the 

 course and results of development would undoubtedly 

 differ in the two cases. 



