8 Journal of Comparative Neurology and PsycJiology. 



In the frog the end-arborizations do not terminate neces- 

 sarily on one muscle fiber. It is not unusual to find that, 

 while the majority of the terminal fibers confine themselves to 

 one muscle fiber and to a certain definite area, one or more fine 

 non-medullated fibrillae pass far beyond the area or to a neigh- 

 boring muscle fiber. So common is this that in well stained 

 preparations one is ever expectant of finding at least traces of 

 such fibrillae. They are often difficult to observe ; but the 

 cause of this is not so much that they do not readily stain, as 

 that it is hard to fix the dye and easy to have it extracted dur- 

 ing the stages subsequent to fixation. 



Such non-medullated fibrillae may be termed ultraterminal. 

 A convenient way to describe them is to classify them accord- 

 ing to the manner in which they end, so far, at least, as such 

 can be traced at present. With this in view, we might describe 

 them as follows : 



(i) Relatively thick, non-medullated fibers which pass to adjacent 

 muscle fibers and divide into endings which resemble more 

 or less closely, though much smaller, the dichotomously 

 branching arborizations from which they spring. These come 

 off very soon after the primary axial fiber loses its medullary 

 sheath and begins to break up (Fig. i ; Fig. 4; Fig. 6). 



(2) Fine fibrillae which pass from the nerve ending into the inter- 

 muscular connective tissue and there cease to be capable of 

 being followed farther (Fig. i; Fig. 3; Fig. 2). 



(3) Very fine non-medulated fibrillae which detach themselves at 

 various points of the terminations and pass to end in adjacent 

 muscle-fibers in one of the following ways : 



a) by getting so faint and so fine that it becomes impossible to 



follow them farther (Fig. i; Fig. 2; Fig. 3). 



b) by terminating at what appears as a much thickened knob 



(similar to Fig. i , B and C) 



c) by breaking up into a plexus from which some at least of the 



fibrillae disappear in the muscle fiber while others continue on 

 (Fig. 2) . 



d) by forming a plexus which enters into close relationship with 



a typical nerve ending (Fig. 3) 



e) by breaking up after a relatively long course to form a small 



localized ending, each termination of which is furnished with 

 a knob (Fig. 6). 



