82 Cc. J.. HERRICK AND ‘G. E,.COGHIDL 
cation from suite to suite and also with a series of large common 
living rooms. Here the individual suites would correspond with 
the separate reflex circuits and the common living rooms to the 
higher correlation centers. 
CONCLUSION 
The general results of this inquiry may now be briefly sum- 
marized and discussed. 
The first response which the Amblystoma embryo can make 
to external stimulation of the skin is a simple avoiding reaction, 
turning the head away from the side touched. This is soon 
followed by an S-shaped reaction of the whole body, and this in 
turn by a simple swimming reaction. In the earliest stages of 
all of these reactions the afferent nervous impulse is transmitted 
through a chain of several neurones to the upper end of the 
spinal cord, thence across the ventral commissure into a de- 
scending efferent or motor tract, which is also composed of a 
chain of neurones. In the earliest swimmers the initial response, 
excited by cutaneous stimulation, is supplemented by a pro- 
prioceptive ‘muscle-sense’ response excited by the muscular 
contraction itself, and thus the rhythm of serpentine locomotion 
is maintained (figs. 5 and 6). At this age the same peripheral 
sensory neurone may serve both as exterceptor and as pro- 
prioceptor (figs. 2 and 3) and on the efferent side of the arc there 
is also a lack of differentiation between the tract neurones and 
the peripheral motor neurones (fig. 3). The only possible re- 
action to stimulation is a total response of the somatic muscula- 
ture—the swimming reaction. 
In the spinal cord of the half grown larva the simple peripheral 
sensory neurones of the earliest stage have been replaced by 
definitive spinal ganglion neurones, among which thpse concerned 
with exteroceptive responses are probably distinct from those 
involved in proprioceptive reactions. Long ascending and long 
descending tracts are differentiated within the spinal cord and 
peripheral motor neurones are now distinct from those of the 
long descending motor tracts. By the elimination of numer- 
' ous synapses, conduction in the spinal cord is much more rapid 
