CHAPTER VIIL 



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 appear- 

 ance 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 proliferate, and the uniform outline of the cord becomes 

 broken (PL xiii. fig. 3). There is formed in this way a small 

 prominence of cells springing from the sum.mit of the spinal 

 cord, and constituting 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 shown in PI. xiii. fig. 2, and the rudiments 

 may there be seen as two club-shaped masses of cells, which 

 have grown outwards and downwards from the extreme dorsal 

 summit of the neural canal and in contact with its walls. The 

 rudiments of the two sides meet at their point of origin at the 

 dorsal median line, and are dorsally perfectly continuous with 

 the walls of the canal. 



» Phil Trans. Vol. 166, p. 175. 



