Hardesty, spinal Netves of the Frog. yj 



the axis cylinder preceeding the myelin sheath by short but 

 varying distances. 



Kolster ('93) in studying the regeneration of fibers follow- 

 ing the section of the nerve, pictures the myelin sheath as be- 

 ing reformed a node at a time, the regenerating axis cylinder 

 preceding the development of the myelin. Accepting the 

 above description that the sheath forms, node by node, the 

 sheath would necessarily cease bluntly and transverse sections 

 stained with osmic acid alone would only show the growing 

 fiber appearing or disappearing suddenly, the axis cylinder por- 

 tion being invisible. 



Whatever the source of the medullary sheath, it was 

 thought highly probable that among the small fibers of the 

 transverse sections here counted, there might be observed some 

 fibers cut at the point at which this sheath was just being ac- 

 quired. In examing the preparations with this object in view, 

 certain difficulties had to be recognized. 



1. The preparations not having been made with this point 

 in mind, in the mounting, good sections rather than serial sec- 

 tions had been chosen. 



2. Since it was desired that the section chosen for count- 

 ing should be one from within a fraction of a millimeter of a cer- 

 tain point along the course of a root or trunk, but few sections 

 were usually mounted. 



3. The average of the decreases in number even in the 

 roots being about i fiber to each 100 in a millimeter of length, 

 it is evident that in the necessarily very thin sections, the op- 

 portunities of observing a disappearing fiber are rare. For 

 these reasons it may be, very few satisfactory cases of a disap- 

 pearing fiber have been observed. One of these is represented 

 in Figs. 1-6, Plate VII. In the search for disappearing fibers, 

 sections were now and then observed which were evidently cut 

 so as to contain only the node of Ranvier. These of course 

 when followed in the series showed in both directions a reap- 

 pearance of the myelin sheath. 



