106 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. 
