88 ELBERT CLARK 



are seen to grow down into the old fiber sheaths of the peripheral 

 stump. All this may take place before there is any marked in- 

 crease in the number of nuclei of the neurilemma sheath and 

 long before embryonic nerve fibers develop. It is further a com- 

 mon observation (as pointed out by Langley and Anderson ('04)) 

 that, unless very special precautions are taken to prevent it, 

 the peripheral stump is invaded by foreign fibers from neigh- 

 boring nerves; and Forsmanns has shown that even macerated 

 brain tissue exerts a chemotactic influence on the outgrowing 

 axis cylinders. 



Clearly then the importance per se of the embryonic nerve 

 fiber in the regeneration of meduUated nerves has been greatly 

 overestimated. 



To summarize briefly, there have been observed in the nerves 

 of this series of fowls which have recovered from a pronounced 

 paralysis of the legs brought on by a prolonged diet of white 

 rice, the following: sections of nerves, taken from the sciatic and 

 its peripheral branches at various times during recovery of the 

 fowl, showed a greater percentage of fibers possessing axis cylin- 

 ders in the sciatic than in its peripheral branches. In regener- 

 ation, a new axis cylinder was acquired by those nerve fibers in 

 which a long series of observations prove that the axis cylinder 

 and myelin sheath had undergone marked degeneration. The 

 large globules and small droplets of degenerated myelin persisted 

 several months after complete recovery of the fowl. Multipli- 

 (cation of the nuclei of the neurilemma sheath and the resulting 

 embryonic nerve fiber were lacking or were of the greatest infre- 

 quency in regenerating as well as degenerating medullated fibers. 

 A new axis cylinder and segments or fragments (jf the old and 

 globules of degenerated myeUn were found together in the same 

 nerve fiber, whose neurilemma sheath showed no increase in its 

 nuclei.- A new axis cylinder, a branch of the same ending in 

 a bulb, fragments of the old axis cyhnder and globules of degen- 

 erated myelin (and with no multiplication of the nuclei of the 

 neurilemma sheath) were all seen in the same portion of a regen- 

 erating fiber. Two axis cylinders in the same fiber and indica- 

 tions of an outgrowing axis cylinder were observed. 



