II 



NERVE DEVELOPMENT 



105 



The His view is concerned primarily with the actual functional 

 nerve -fibres. As regards the primitive sheath (Gray Sheath of 

 Schwann), in which these 

 fibres are enclosed, the 

 His view regards it as 

 being formed by mesen- 

 chyme cells which apply 

 themselves to, and spread 

 out over, the surface 

 of the originally naked 

 nerve-fibre. 



(2) The Balfour 

 ViEW(Cell -chain theory). 

 —While Schwann(1839) 

 long ago described the 

 multicellular structure of 

 nerve-trunks in the foe- 

 tuses of mammals, it was 

 F. M. Balfour (1876) who 

 really founded the view 

 that the nerve - trunk 

 arises in development 

 from a chain of cells. 

 Balfour found in Elasmo- 

 branch embryos that the 

 nerve -trunk was repre- 

 sented by a chain of 

 cells in early stages (Fig. 58, v.r), and similar observations have been 

 made by subsequent observers. According to this view the whole nerve- 

 trunk is multicellular in origin, the cells not only forming the sheath of 

 the nerve-trunk but also giving rise to the nerve -fibrils which come 

 into existence traversing the cellular strand from end to end. 



On the question of the origin of the cells which constitute the 

 nerve-rudiment opinions vary. Most supporters of this view have 

 regarded them as having emigrated from the spinal cord {e.g. Balfour, 

 van Wijhe, Dohrn) : while others (Kolliker) have looked on them 

 as mesenchymatous in nature. Sedgwick took this latter view and 

 as he regarded the mesenchyme as a continuous syncytium, the 

 bridges connecting the cells being primitive — persisting from the 



Fig. 58. — Section through the dorsal part of the trunk 

 of a Torpedo embryo. (From Balfour's Embryology.) 



d.r, dorsal root; g, spinal ganglion; my, myotome ; N, noto- 

 chord ; n, nerve-trunk ; no, cavity of spinal cord ; v.r, ventral 

 root. 



the surface of the cell and its processes. We know from the recognized unreliability 

 of the method that the occurrence, or not, of this precipitation is liable to be decided 

 by extremely delicate chemical differences. We know further that the axis cylinder, 

 however it arose in development, is morphologically and physiologically a prolonga- 

 tion of the cell-body (ganglion-cell), and therefore that its metabolism is under the 

 control of the nucleus of that cell-body. The individuality of the cell and its pro- 

 longation, due to the metabolic control by its own special nucleus, is probably quite 

 enough, in itself, to account for a chemical character of its surface sufficiently different 

 from that of its neighbours to influence the precipitation without there being, as the 

 neurone theory assumes, any absolute discontinuity. 



