DEVELOPMENT OF REFLEX MECHANISMS (iv 
oblongata, except the extreme caudal end, show essentially the 
same pattern of functional localization of peripheral sensory root 
fibers. 
A typical section through the middle of the oblongata is shown 
in figure 8, and the ensemble of these roots as projected upon the 
lateral surface of the oblongata is indicated diagrammatically 
in figure 9. 
The bundles of sensory root fibers, as shown by figure 8, make 
up most of the white substance of the dorsal half of the medulla 
oblongata, save for two strong correlation tracts (tr.a and tr.b.). 
The ventral half of the white substance contains the motor 
roots (a ventro-lateral visceral series and a ventro-medial somatic 
series) and numerous long correlation tracts between the oblon- 
gata and other parts of the central nervous system. 
In these larvae the peripheral sensory and the peripheral 
motor neurones appear to be ag clearly differentiated into dis- 
tinct functional systems, each with its own type of highly special- 
ized end-organ, as in higher vertebrates; and indeed the arrange- 
ment of these functional systems in the cranial nerve roots is 
essentially similar throughout the vertebrate series. But the 
central correlation neurones by which these peripheral elements 
are put into physiological connection exhibit here an arrange- 
ment which, so far as hitherto described, is unique among the 
vertebrates. The specificity of function which is so characteristic 
of the peripheral neurones is here no longer in evidence, except 
in a very general way. This is illustrated by the relations shown 
in figure 10. The neurone shown on the right side of this figure 
sends one main dendrite downward into the spinal V tract, where 
it engages collaterals from root fibers of the general cutaneous 
system. Another main dendrite passes laterally into the fascic- 
ulus solitarius, where it engages collaterals from the gustatory 
and other visceral sensory root fibers which compose this fascic- 
ulus. The axone of this neurone may enter the uncrossed 
secondary visceral tract (d7.v.a.), or it may decussate to enter a 
_ crossed ascending path (¢r.b.t.), or in some cases it appears to 
divide and send a branch into each of these tracts, as shown in 
the figure. The left side of figure 10 illustrates a neurone lying a 
