1 88 DEVELOPMENT OF THE SPINAL NERVES 



the posterior roots of the spinal nerves grow out from the spinal 

 cord : it is the rudiment of the seventh or facial nerve. The 

 section behind this (fig. M II), still in the region of the ear, 

 has no trace of a nerve, and thus serves to shew the early dis- 

 continuity of the posterior nerve-rudiments which arise from 

 the brain. 



I have as yet failed to detect any cranial anterior roots like 

 those of the spinal nerves 1 . The similarity in development be- 

 tween the cranial and spinal nerves is especially interesting, as 

 forming an important addition to the evidence which at present 

 exists that the cranial nerves are only to be looked on as 

 spinal nerves, especially modified in connexion with the changes 

 which the anterior extremity of the body has undergone in 

 existing vertebrates. 



My results may be summarized as follows : 



Along the extreme dorsal summit of the spinal cord there 

 arises on each side a continuous outgrowth. 



From each outgrowth processes corresponding in number 

 to the muscle-plates grow downwards. These are the posterior 

 nerve-rudiments. 



The outgrowths, at first attached to the spinal cord through- 

 out their whole length, soon cease to be so, and remain in con- 

 nexion with it in certain spots only, which form the junctions 

 of the posterior roots with the spinal cord. 



The original outgrowth on each side remains as "a bridge, 

 uniting together the dorsal extremities of all the posterior rudi- 

 ments. The points of junction of the posterior roots with the 

 spinal cord are at first situated at the extreme dorsal summit of 

 the latter, but eventually travel down, and are finally placed on 

 the sides of the cord. 



After these events the posterior nerve-rudiments grow 

 rapidly in size, and become differentiated into a root (by 

 which they are attached to the spinal canal), a ganglion, and 

 a nerve. 



The anterior roots, like the posterior, are outgrowths from 

 the spinal cord ; but the outgrowths to form them are from the 



1 [May 1 8, 1876. Subsequent observations have led me to the conclusion that no 

 anterior nerve-roots are to be found in the brain.] 



