258 
G. E. Coghill 
upon the principle of cephalization as correlated with the organs of 
special sense; but these early movements of the embryo show, that 
so far as functional development is concerned, the most primitive 
centralization of the nervous system, ontogenetically, is in direct 
response to the demands of the motor system in its relation to loco- 
motion, while the sensory system involved is not the special sensory 
but the most primitive, diffuse, exteroceptive field. It remains to 
locate exactly this primitive center of the cerebro-spinal system by 
correlated anatomical and experimental studies; but from the 
experiments alone, this center would seem to be in close relation 
to the cephalic musculature of the trunk. This is inferred par- 
ticularly from the fact that a flexure in response to a touch on the 
tail bud begins in the head region and progresses caudad and is the 
same in form (without reference to the initial direction of the 
movement) as the flexure that follows stimulation of the head. All 
movements, then, regardless of the point of stimulation, must 
emanate from the same center. Into the center all impulses 
would seem to flow in order to be directed in such a way upon the 
musculature of the trunk as to give rise to locomotion. Clearly the 
development of an eye or ear as such in its earliest functional con- 
dition has no part in determining this region of centralization. 
The controlling factor in this centralization is the motor system: 
a cephalization in response to the prepotency of the requirements 
of effectors and not in response to the demands of the cephalic 
receptive fields. 
Phylogenetically, then, the most primitive cephalization of the 
nervous system may have occurred, also, in response to the 
demands for locomotion and have given rise to a center of control 
in the region corresponding to the lower portion of the myelen- 
cephalon or the upper portion of the medulla spinalis. Quite in 
harmony with this suggestion is the convincing evidence that 
Johnston® presents for the migration caudad of the afferent roots of 
the cranial nerves. Such a change in their course would lead them 
more directly into this primitive locomotor center. Upon this 
hypothesis, also, the economy of the arrangement of the special 
cutaneous nerves of fishes and amphibians is obvious. It is not to 
be supposed that the cephalization of the locomotor effectors is, in 
any respect, a direct cause of the cephalo-caudal migration of the 
^The Nervous System of Vertebrates, chapter iii. 
