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. 
