NERVOUS SYSTEM OF THE VERTEBRATA. 



453 



It is extremely difficult to decide whether the permanent attachment of 

 the posterior nerve-roots to the spinal cord is entirely a new formation, or 

 merely due to the shifting of the original point of attachment. I am inclined 

 to adopt the former view, which is also held by Marshall and His, but may 

 refer to fig. 269, shewing the roots after they have become attached to the 

 side, as distinct evidence in favour of the view that the attachment simply 

 becomes shifted, a process which might perhaps be explained by a growth of 

 the dorsal part of the spinal cord. The change of position in the case of 

 some of the cranial nerves is, however, so great that I do not think that it is 

 possible to account for it without admitting the formation of a new attach- 

 ment. 



The anterior roots of the spinal nerves appear somewhat 

 later'than the posterior roots, but while the latter are still quite 

 small. Each of them (fig. 269 ar) arises as a small but distinct 

 conical outgrowth from a ventral corner of the spinal cord, 

 before the latter has acquired its covering of white matter. 

 From the very first the rudiments of the anterior roots have a 

 somewhat fibrous appearance and an indistinct form of peripheral 



? 



mp w 



V 



FIG. 268. SECTION THROUGH THE DORSAL REGION OF A PRISTIURUS EMBRYO. 

 pr. posterior root; sp.g. spinal ganglion; n. nerve; x. attachment of ganglion to 

 spinal cord; nc. neural canal; ;///. muscle-plate; ch. notochord ; i. investment of 

 spinal cord. 



termination, while the protoplasm of which they are composed 

 becomes attenuated towards its end. They differ from the 

 posterior roots in never shifting their point of attachment to the 

 spinal cord, in not being united with each other by a commissure, 

 and in never developing a ganglion. 



