SPINAL CORD REGENERATION. II 431 
order C, D, HE, F. They are followed by the appearance of 
sensory fibers, first from D and later from F. 
If the segment DE is reversed in position, as shown in figure 3, 
III, the two originally cephalic stumps, C and E, are brought into 
apposition, as are the two originally caudal stumps, D and F. 
Two possibilities for the outgrowth of nerve fibers from the stumps 
in this position present themselves. Either (1) the original polar- 
ity of the neurones and the fibers, being an inherent function of 
the cells themselves, will be maintained in their reversed position 
and the surfaces C, D, FE and F will exhibit the same kind of 
fibers as they would have done in their original position, or (2) 
the polarity of the neurones and their fibers, if a function of their 
position, would be reversed with the reversal of their position, 
so that D would show only motor fibers growing from it and EH 
both motor and sensory. 
Serial sections of a considerable number of embryos in this 
stage have been available and they demonstrate that the first of 
these possibilities is the one which actually occurs. The posi- 
tion of the cord segment is reversed but the polarity of the 
neurones within that segment is unaltered. The surface of the 
stump C exhibits only nerve fibers which are the descending 
processes of motor neurones. The surface D presents fibers 
which from their morphological character and from their posi- 
tion must be ascending processes of both motor and sensory 
neurones. Such a condition is also exhibited by the wound 
surface F, while E exhibits only motor fibers. 
One most noticeable characteristic of the further development 
of these nerve fibers is that they show a marked tendency to 
avoid entering the wound surface opposite them. While in the 
simple cord severing experiments, it was quite rare to find the 
ascending branches of the sensory neurones from the caudal 
stump 6b wandering away from and not entering the cephalic 
stump A, in the embryos with a reversed piece of the cord, it is 
quite rare to find any sensory fibers from F entering D or vice 
versa. The motor fibers also exhibit this antagonism, but to a 
less degree, so that at least partial motor union between the two 
adjacent surfaces is fairly common. It is this avoidance of the 
