MORPHOLOGY OF EYE MUSCLE NERVES 13 



In correlation with this movement of medullary protoplasm 

 there occurs a similar movement of the protoplasm of the cells 

 of the sclerotomic portion of the somite, which is inaugurated 

 by an extension of amoeboid processes toward the neural tube 

 (figs. 2 and 3) followed in later stages by a movement of entire 

 cell bodies. In many cases, but not invariably, the movement 

 of the sclerotome cells precedes that of the medullary cells. The 

 protoplasmic material derived from the two sources in excep- 

 tional cases unites to form a protoplasmic strand or plasmo- 

 desm in which it is difficult to distinguish ectodermal and meso- 

 dermal constituents, as is shown in figures 8 and 9. This dif- 

 ficulty obtains, however, only in the earliest stages, if it occurs 

 at all. Even in such cases the sclerotome cells soon separate 

 to form a loose mesenchyma of lightly staining and much vacuo- 

 lated cells, while the medullary portion persists as a compact 

 strand of deeply staining protoplasm which qtiickly takes on a 

 fibrillar appearance and also becomes cellular through a process 

 of cellular migration from the neural tube (figs. 11 and 12). 



Evidence that the cells of the somatic motor nerve anlagen 

 of Squalus are chiefly, if not exclusively, of medullary origin has 

 been given in an earlier paper (Neal '03) and need not be reviewed 

 in detail at this time. Their medullary derivation seems suffi- 

 ciently attested by evidence of continued migration in succes- 

 sive stages; by their close apposition to the fibrillar bundle or 

 their inclusion within the fibrillar portion of the nerve anlage 

 (fig. 12); by the change in the contour of the neural tube in 

 successive stages as seen in cross-sections; and by the relations 

 of the outer limiting membrane of the tube to the nerve anlage. 

 If mesenchymatous cells are added to the nerve anlage at all, 

 it appears to be in later stages of histogenesis only. 



While spinal somatic motor nerve anlagen are primarily proto- 

 plasmic and non-fibrillar this stage is quickly passed and, as 

 the anlage extends ventrad along the median surface of the 

 myotome between the myotome and sclerotome of the somite, 

 it assumes a fibrillar appearance. That these fibrils are neuro- 

 fibrillar in the Apathy sense is proven by the fact that they 

 stain intensely in either Cajal or Paton preparations. Evidence 



