444 DAVENPORT HOOKER 
DISCUSSION 
From the foregoing résumé of the details of the experiments, 
it is evident that when a portion of the spinal cord taken from 
the cervical or upper thoracic region of the frog embryo is re- 
moved, turned end-for-end and grafted into position, the piece 
as a whole retains its original polarity. This is what one would 
expect from the results obtained by Spemann (712). The fact 
that the reversal of position does not affect the original polarity 
of the piece reversed is amply demonstrated by the persistence 
of the medullary cavity at the originally cephalic end of the 
middle piece. Furthermore, the manner in which the develop- 
ing nerves grow out from the cut ends of the reversed middle 
piece demonstrates that in the beginning at least the reversal 
of position does not affect the polarity of the elements contained 
within the piece reversed. The nerve processes arising from these 
ends are exactly the same in kind as those which would have arisen 
if the piece had remained in its original position. This fact 
complicates the ensuing attempts to re-establish the continuity 
of the cord. After simple severing of the cord, the descending 
processes of the motor neurones situated at the various levels of 
the cord began their development in normal orientation to the 
cord as a whole. There is in reality no true regeneration of 
the individual elements, inasmuch as the operations are carried 
out before the time when the nerves normally begin to develop. 
In consequence of this fact, these processes are subjected to no 
other abnormal condition than the necessity of traversing an 
area filled wtih connective tissue. The same is true as regards 
the ascending processes of the sensory neurones. In the embryos 
under discussion, on the other hand, the normal relationship 
between the direction of growth of all the processes and the 
antero-posterior axis of the embryos has been completely upset 
so that the nerve fibers which were originally descending pro- 
cesses grow in an ascending direction and vwice versa. AS we 
have seen, this brings together at the cephalic wound surface a 
series of nerve fibers growing in opposite directions which are all 
descending processes and at the caudal wound a number growing 
