84 C. J. HERRICK AND G. E. COGHILL 
type of total reaction and the gradual perfection of a great 
variety of individual adaptive movements, for each of which a 
particular chain of neurones is set apart. Rapid conduction, 
through each of these circuits is then facilitated by the elimi- 
nation of unnecessary synapses and the closer articulation of the 
residual neurones. From this it follows that the ‘typical’ two- 
neurone, short-circuit connection between dorsal and ventral 
root fibers, as illustrated by figure 1, appears late in development 
and is not to be regarded as a primitive form. In fact, all of the 
long correlation pathways of the central nervous system, appear 
to develop relatively late in the ontogeny out of more complex 
chains composed of many more neuronic units. In this connec- 
tion one is reminded that in the human brain the longest path of 
all, the pyramidal tract, is one of the last to mature. 
How far the embryological sequence shown in the development 
of these pathways in the amphibian brain should be interpreted 
as evidence of the phylogenetic sequence, it would at present be 
premature to affirm positively. But it seems probable to us that 
’ the relations found in amphibian larvae are in many respects 
primitive; and this is in accord with the known form of connection 
of the nervous elements in the simplest types of nervous system 
and with the prevailing belief that every form of central nervous 
system has arisen by the concentration of an originally diffuse 
and relatively equipotential peripheral ganglonated plexus in 
the interest of an integration of all bodily functions. Parallel 
with this integrative process there was a progressive individuation 
of particular reflex circuits and their segregation out of the pri- 
mordial general nervous matrix. Special correlation centers 
must then be developed in order that the primary integrative 
action of the nervous system may not be impaired in this process 
of individuation of its parts; and the more complex the particular 
functions of the parts, the more important become the corre- 
lation centers. Thus arose the great suprasegmental apparatuses 
(cerebellar and cerebral cortex) superposed upon the more ancient 
reflex systems of the brain stem. 
Finally, we would urge that the factors operating in either 
the ontogenetic or the phylogenetic differentiation of the func- 
