72 C. J. HERRICK AND G. E. COGHILL 
From these observations it follows that the earliest reflex 
responses to external stimulation require a rather complex 
chain of neurones, whose relations are quite different from those 
of the shortest definitive reflex arc of the adult animal. 
In this arrangement we would emphasize the following features: 
(1) the fact that a single peripheral sensory neurone may be 
excited either by an exteroceptive stimulus applied to the skin 
or by a proprioceptive stimulus (‘muscle sense’) arising from the 
contraction of the myotome during the act of swimming; (2) 
the fact that the peripheral motor neurones are not yet separately 
differentiated away from those of the central motor tract; (3) 
that the reflex path required for the type of crossed reaction 
which is first to appear is a relatively long one, all of the afferent 
impulses having to reach the upper end of the spinal cord before 
they can be transferred across the mid-plane into the efferent 
apparatus; (4) that the individual neurones of this circuit are 
relatively short and that many synapses are interpolated in the 
pathway. 
' In the actual operation of this mechanism it appears that a 
somatic movement may ‘be initiated by a single external tactile 
stimulus (say at S, fig. 5) which is transmitted inward through 
the cutaneous dendrite of a dorsal giant cell (figs. 2 and 3) and 
that this movement takes the form of a swimming reflex whose 
rhythm begins at the head end of the body and is transmitted 
backward through the myotomes of the side opposite to the one 
stimulated. The first stage in this movement is a bending of 
the head away from the side touched (fig. 5). This curvature 
of the body then moves backward, involving the successive 
contraction of additional myotomes, and after several myotomes 
of the upper part of the trunk have been involved, the muscular 
branches of the dorsal giant cells are excited by the contraction 
of their respective myotomes (myotomes 11 to 16 on the left 
side of fig. 6), thus exciting a proprioceptive reaction. This 
stimulus is likewise transmitted to the upper end of the spinal 
cord, passed over to the opposite side and into myotomes lying 
in front of those now in contraction (myotomes 1 to 6 on the 
right side of fig. 6). The effect is to turn the head to the opposite 
