CHAPTER VIII. 



DEVELOPMENT OF THE SPINAL NERVES AND OF THE 

 SYMPATHETIC NERVOUS SYSTEM. 



The spinal nerves. 



THE development of the spinal nerves has been already 

 treated by me at considerable length in a paper read before 

 the Royal Society in December, 1875*, and I have but little 

 fresh matter to add to the facts narrated in that paper. The 

 succeeding account, though fairly complete, is much less full 

 than the previous one in the Philosophical Transactions, but a 

 number of morphological considerations bearing on this sub- 

 ject are discussed. 



The rudiments of the posterior roots make their appearance 

 considerably before those of the anterior roots. They arise 

 during stage I, as outgrowths from the spinal cord, at a time 

 when the muscle-plates do not extend beyond a third of the way 

 up the sides of the spinal cord, and in a part where no scattered 

 mesoblast-cells are present. They are formed first in the 

 anterior part of the body and successively in the posterior parts, 

 in the following way. At a point where a spinal nerve is about 

 to arise, the cells of the dorsal part of the cord begin to pro- 

 liferate, and the uniform outline of the cord becomes broken 

 (PI. 14, fig. 3). There is formed in this way a small prominence 

 of cells springing from the summit of the spinal cord, and con- 

 stituting a rudiment of a pair of posterior roots. In sections 

 anterior to the point where a nerve is about to appear, the nerve- 

 rudiments are always very distinctly formed. Such a section is 

 shewn in PI. 14, fig. 2, and the rudiments may there be seen 



1 Phil. Trans. Vol. 166, p. 175. [This Edition, No. vni.] 



