254 THE ORIGIN OF THE NERVOUS SYSTEM 
extends both anteriorly and posteriorly, until commis- 
sures are present at all levels posterior to the optic 
region. 
The first motor response to tactile stimulation of 
regions posterior to the head is the contraction of 
muscles of the side opposite to that stimulated, first in 
the more anterior region just behind the head and later 
at more posterior levels, and the reflex mechanism indi- 
cated in Figure 70 constitutes the anatomical basis of 
this type of response. At this stage and in this the 
earliest reflex mechanism of the cord it is the upward 
path which is uninterrupted by synapses, while the 
downward path shows frequent interruptions of this 
kind and is apparently somewhat later in its develop- 
ment. In later stages the giant cells disappear, and 
other reflex mechanisms develop, until the definitive 
condition is attained. 
These different characteristics of the upward and 
downward paths in the different vertebrate groups and 
in different stages of development of the individual must 
of course be dependent on differences in physiological 
conditions of some sort. The experimental data at 
hand concerning the physiological gradients in the 
vertebrates serve, I believe, to throw some light on 
these changes. It has already been pointed out (chap, 
viii) that in all segmented animals investigated, including 
annelids as well as vertebrates, a second region of high 
susceptibility and rapid growth arises early in develop- 
ment and gives rise to most or all of the postcephalic 
region. The presence of this second region of high 
metabolic rate has been demonstrated in fishes and in 
the chick by Hyman, whose work along this line is now 
