SPINAL CORD REGENERATION. II 443 
has excited them to contraction. This contraction may have 
acted as a proprioceptive stimulus to the nervous system of the 
head and tail region which caused in turn a contraction of the 
opposite side of the body. 
It is therefore to be noted that apparently the activity of the 
middle piece is responsible for the early movements of embryos 
in which this middle piece has been reversed. In a few indi- 
viduals the middle piece sloughed out immediately after opera- 
tion and these embryos continued their development with this 
wide gap in the back. Such embryos never exhibited even an 
approach to a swimming reaction. They were never able to 
produce locomotion over the most limited distances. Indeed it 
can be said that the middle piece is the only one which exhibits 
a true § reaction. 
On the other hand, it must be remarked that for a normal 
swimming movement some sort of nervous connection between 
the cut wound ends is essential. From a careful study of the 
stages of regeneration in the spinal cord in correlation with the 
type of swimming movement exhibited, it seems that motor 
connections are alone essential for a normal swimming move- 
ment. It is further apparent that the ascending motor processes, 
which are the ones which bridge the caudal cut, are capable of 
acting as typical descending processes and it is evident that 
motor stimuli from the middle piece are transmitted along these 
fibers to the tail portion of the embryo. Conversely, it is also 
apparent that the descending processes growing out from the 
cephalic portion of the piece may also change their functions 
and transmit stimuli to the head region of the embryo. In this 
sense there is a reversal of the polarity of the neural elements 
contained within the reversed portion of the cord. On the other 
hand, there is considerable question as to the specificity of these 
two types of processes. 
The establishment of sensory connections between the isolated 
portions of the cord is long delayed in the embryos under dis- 
cussion and seems to play little or no réle in the development 
of the swimming movement. 
