NERVOUS SYSTEM AND OTHER PARTS 119 



or more active region A becomes a factor in determining 

 through transmission of excitation the physiological con- 

 dition of another less irritable or less active region B, 

 in so far is A independent of B. Evidently then the 

 high end of a gradient must be, at least to a large extent, 

 independent of other levels, and in bilateral forms not 

 only must the anterior region be largely independent of 

 more posterior levels, but the median ventral (inverte- 

 brates) or the median dorsal (vertebrates), so far as it 

 represents the high end of the symmetry gradients, must 

 be relatively independent of more lateral regions. 



Such a relation, if it actually exists, must appear in 

 the order of development and the functional relations of 

 parts along an axis. The so-called law of anteroposterior 

 development is merely a statement of this relation for 

 the longitudinal axis. Development and differentiation 

 begin anteriorly or apically and progress posteriorly or 

 basally because the anterior or apical region, being more 

 intensely active than other regions, is so far as they are 

 concerned self-determining, at least in large measure, and 

 each body level is also to some extent self-determining 

 with respect to more basal or more posterior levels and 

 at the same time to some extent determined by more 

 apical or anterior levels (Figs. 8-12). Similarly the 

 median region is to some extent self-determining with 

 respect to the lateral, and more lateral levels are to some 

 extent determined by more median. In plants the 

 growing tip arises first in the development of an indi- 

 vidual axis, whether stem or root, and is manifestly 

 self-determined with respect to other parts, since it may 

 arise in their complete absence, and other parts are sub- 

 ordinate to and determined by it. 



