584 Jourjial of Comparative Neurology and Psychology. 



relatively slender and thickens greatly as it enters the myotome. 

 Only a short part of this branch is drawn. It continues in the 

 next adjacent section parallel with the other fibers and sends off 

 a great number of branches which, if inserted here, would have 

 rendered the drawing very obscure. 



These two forms of endings and their distribution doubtless 

 indicate the beginning of specialization in the myotomes. As the 

 mesal portion of the myotome is the first part to develop muscle 

 fibrillae in the ontogeny and as this part becomes the muscle plate 

 in the embryos of higher vertebrates, so here the mesal part of 

 the myotome is supplied with special motor endings, while the 

 lateral and greater part has only the simple end branches. In the 

 phylogeny, with the development of hard parts in the vertebral 

 column, the adjacent mesal parts of the myotomes come to be 

 especially efficient in body movements and so become specialized 

 skeletal muscles. The beginning of this is at least foreshadowed 

 in the lamprey. 



Segmental relations of the ventral nerves. — In figs. 14 and 15 

 it is clearly shown that the motor fibers of both types may inner- 

 vate two myotomes. These nerves are not impregnated in a 

 sufficiently large number of segments or of specimens to enable 

 me to say whether the distribution of one fiber to two myotomes 

 is common, but the fact that the majority of the fibers which are 

 impregnated send branches to two myotomes is sufficient indica- 

 tion that it is at least a normal arrangement. The significance 

 of this seems to the writer to lie not so much in its bearing on 

 questions of segmentation as upon the question of the factors 

 determining the distribution of nerve fibers and directing them to 

 their endings. There seems to be no definite or constant arrange- 

 ment of these motor fibers. They pass in a haphazard fashion 

 to one or two myotomes, branch once, twice or three times, etc. 

 In studying the peripheral nerves of Amphioxus with methylene 

 blue I gained the general impression that the nerves in that animal 

 showed still less regard for segmental relations. The sensory 

 nerves run more or less straight ventrad over several obliquely 

 placed myotomes and present endless variations in the way in 

 which they reach the same general area of distribution. The 

 obstacle in the way of Hensen and his followers accepting His's 

 theory of the outgrowth of nerve fibers has been that they can not 

 see how peripheral nerve fibers can find their way out from the 



