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 



