86 ELBERT CLARK 



All these observations speak strongly for a new axis cylinder 

 in recovery; the greater percentage of fibers with axis cylinders 

 in the sciatic than in its peripheral rami argues also for an out- 

 growth of the axis cylinder. Further evidence of outgrowth of 

 the axis cylinder was soon obtained. Before I was able to con- 

 firm the first observation that a new axis cylinder and segments 

 of the old were to be found in the same fiber at the same time, 

 another fiber was observed in the same preparation, in which 

 growth activity was apparent. This fiber is shown in figures 

 27 and figures 28 and 29 (photomicrographs) all of the same fiber. 

 It will readily be seen that h is an outgrowing branch of the 

 axis cylinder a. That a is a new axis cylinder is proven by the 

 presence of a fragment of the old axis cylinder, a', (fig. 27 and 

 fig. 29) between the new, a, and its branch, b. Both axis cylinder 

 and branch stained equally well and much better than the rem- 

 nants of the old. An end bulb is seen on the tip of the branch. 

 It might be pointed out here that Cajal and Marienesco have 

 observed a similar branching of the outgrowing axis cylinder in 

 medullated nerves after section. These branches, of which there 

 may be one or more to each fiber, often take an abortive course 

 and have been observed to grow in a recurrent direction up the 

 central stump. Whether, in the present case, b is an outgrowth 

 from a or whether at an earlier stage both were outgrowing buds 

 of approximately equal size is purely a matter of speculation. 

 It should be added that in cross-sections old sheaths were ob- 

 served which contained two axis cylinders of approximately equal 

 size, each surrounded by a secondary sheath of its own. But 

 more frequently one is much larger and occupies a more cen- 

 tral position than the smaller which may be located quite near 

 the periphery of the sheath. Figures 25 and 26 from the sciatic 

 of fowl No. 54, show in transverse section two axis cyhnders in 

 the same nerve sheath, and apparently in the same portion of 

 the fiber that was formerly occ\ipied by the old axis cylinder. 

 Whether the zone around each represents a newly acquired mye- 

 hn sheath I have not determined. Nuclei, however, have not 

 been observed in this zone. 



