1 86 DEVELOPMENT OF THE SPINAL NERVES 



both ends. The processes of the cells are at this stage very 

 difficult to observe : figs. la, I b, I c represent three cells pro- 

 vided with them and placed in the positions they occupied in 

 the ganglion. 



The relatively very small amount of protoplasm in com- 

 parison to the nucleus is fairly represented in these figures, 

 though not in the drawing of the ganglion as a whole. In the 

 centre of each nucleus is a nucleolus. 



Fig. I b, in which the process points towards the root of 

 the nerve, I regard as a commencing nerve-fibre : its more elon- 

 gated shape seems to imply this. In the next stage special 

 bundles of nerve-fibres become very conspicuous in the gan- 

 glion. The long diameter of an average ganglion-cell is about 

 Ysm f an inch- The whole ganglion forms an oval mass, well 

 separated both from the nerve-root and the nerve, and is not 

 markedly continuous with either. On its outer side lies the 

 downward process of the nerve-root before mentioned. 



The nerve itself is still, as in the last case, composed of cells 

 which are larger and more elongated than either the cells of the 

 root or the ganglion. 



The condition of the anterior root at this stage is hardly 

 altered from what it was ; it is composed of very small cells, 

 which with haematoxylin stain more deeply than any other cell 

 of the section. A figure of it is given in I II. 



Horizontal longitudinal sections of this stage are both easy 

 to make and very instructive. On PI. 23, fig. K I is represented 

 a horizontal section through a plane near the dorsal surface 

 of the spinal cord : each posterior root is seen in this sec- 

 tion to lie nearly opposite the anterior extremity of a muscle- 

 plate. 



In a more ventral plane (fig. K 11) this relation is altered, 

 and the posterior roots lie opposite the hinder parts of the 

 muscle-plates. 



The nerves themselves are invested by the hyaline mem- 

 brane spoken of above ; and surrounding this again there is 

 present a delicate mesoblastic investment of spindle-shaped cells. 



Longitudinal sections also throw light upon the constitu- 

 tion of the anterior nerve roots (vide fig. K II, ar). In the two 

 segments on the left-hand side in this figure the anterior roots 



