Johnston, Nerves of Petromyzonts. 585 



central nervous system to the organs which they should innervate. 

 To remove this obstacle it is only necessary to recognize that 

 peripheral fibers do not, "unerringly" as Hensen said, find their 

 way where they "should" go. Recent studies of regeneration 

 give evidences of this and the facts here presented have, I believe, 

 the same significance. In these lowly organized vertebrates the 

 nerve fibers push out much as the pseudopodia of a protozoon 

 are thrust out, and the word haphazard can be applied to the 

 nerve fibers with whatever truth can be claimed for it when applied 

 to pseudopodia. As the pseudopodia of a motor neuroblast grow 

 out they are directed by the forming organs of the body and per- 

 haps by chemical influences. It appears that they do not always 

 take the same course or reach the same end. They go sometimes 

 to one myotome, sometimes to two; sometimes the greater part 

 of a fiber remains in the first myotome which it enters; sometimes 

 it gives ofi^ smaller branches to this myotome and runs through 

 it to enter a second. It should be noticed that in the myotome 

 stage of body musculature about the only thing necessary in the 

 way of definite and constant innervation is a general bilateral 

 symmetry. All that is secured is an alternating contraction of the 

 muscles of the two sides passing along the body in waves, giving 

 the fish the undulating movement by which it swims. As the 

 muscles become specialized in the phylogenetic series — and the 

 same is true of other organs — the influences directing the course 

 of nerve fibers as they grow out increase in definiteness to keep 

 pace with the evolution of the organism. Indeed, this is one 

 factor upon which survival would depend. At no stage of evolu- 

 tion, however, so far as the writer can see, is it necessary to suppose 

 that nerve fibers should be unerringly directed to their proper 

 destinations. Why should not some nerve fibers go astray like 

 sheep and be lost } Why should nerve fibers be exempt from the 

 otherwise universal law, the law of occasional failure ? Does not 

 the method of trial and error hold here ? 



ENDINGS OF MUSCLE SENSE. 



Occasionally there are found in the intermuscular septa free 

 endings of fibers of the dorsal spinal nerves. One is shown in 

 fig. 14. These fibers seem to be in the proper position to serve 

 the muscle sense. Might not these fibers be stimulated by the 



