SPINAL CORD REGENERATION. II 447 
its shortness there is not sufficient length to give a marked arc. 
For the same reason there is no true § reaction in the head re- 
gion. The tail region exhibits all the above phases except that 
the S reaction is very much later in making its appearance than 
is the case with the middle section of the body. 
The reactions in the middle piece of the body can not be lo- 
calized in any portion of it, either as regards the sensitivity of 
the receptors or the functioning powers of the effectors. We 
must therefore consider that this region of the frog embryo, 
about one mm. in length and extending from the caudal end of the 
medulla backward, is a unit not only in its reaction to stimuli, 
but also as regards the development of the primary nervous 
connections within the cord. It would seem then that in the 
frog embryo there is a larger area in which the primary nervous 
connections are developed simultaneously than is found to be 
the case in Amblystoma (Coghill). 
SUMMARY 
1. Reversed portions of the spinal cord heal per primum when 
the edges of the cut have been carefully apposed. When they 
have not been apposed, the wounds may heal according to the 
same principles and by passing through the same stages as do 
wounds caused by simple severing of the cord. 
2. No healing has been obtained in embryos of a later stage 
than that having a fairly well developed tail bud. 
3. The reversed portion of the spinal cord retains its original 
polarity. 
4. The neurones begin their development in normal orienta- 
tion to the reversed piece, but the direction of the transmission 
of stimuli is apparently reversed at a later time. Their mor- 
phological polarity is therefore unaffected by their reversal in 
position but adaptation causes a subsequent reversal of the 
functional polarity. 
5. Embryos in which a portion of the spinal cord has been 
reversed are not as a rule as viable as those in which the cord 
was simply severed. 
