74 DEVELOPMENT OF THE NERVES. 



to the dorsal aspect of the neural tube, but that attachment becomes subsequently 

 lost, and they then form completely isolated portions of epiblast, composed of oval 

 cells, and lying at the side of the embryonic cord between it and the muscle plates 

 of the protovertebrse (fig. 84). These are the rudiments of the posterior-root 

 ganglia. The remainder of the neural crest disappears : at least in most vertebrata. 

 Some little time after the separation of these ganglion-rudiments, the ventral or 

 anterior roots of the spinal nerves begin to grow out from the ventro-lateral aspect of 

 the neural tube. They were originally described by Balfour in elasmobranchs, as 

 forming bud-like outgrowths from the neural epiblast, the outgrowths being com- 

 posed of spindle-shaped cells (fig. 84, ar). But according to the recent and extended 



ch 



Fig. 84. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A TORPEDO EMBRYO. (Balfour.) 



pr, g, n, spinal ganglion rudiment ; ar, anterior root ; ch, notocliord ; nc, neural canal ; mp, muscle- 

 plate. 



Fig. 85. SECTION OF THE VENTRO-LATERAL ANGLE OF THE SPINAL CORD OF A .PRISTIURUS EMBRYO 



SHOWING THE OUTGROWTH OF AN ANTERIOR ROOT- RUDIMENT. (His.) 



a.r, axis cylinder processes of neuroblasts, forming the anterior root ; g, germinal cells in innermost 

 part of wall of neural canal. 



observations of His in various classes of vertebrates, what actually grow out to form 

 the anterior roots, are the fibrous prolongations (axis-cylinder processes) of neuro- 

 blasts (v. antea, p. 58), which processes converge to the point of exit of the root 

 and penetrate gradually into the adjoining mesoblast (fig. 85, a. r), where they 

 come into close contact with the previously formed ganglion rudiments of the pos- 

 terior roots. 1 



The fibres of the posterior roots are developed, according to His, as processes 

 from the oval cells of the ganglion rudiment. These cells are in fact neuroblasts, 

 and from either end of each cell, which is thus rendered bipolar (fig. 8G), a process 

 becoming eventually the axis-cylinder of a nerve-fibre grows out, one towards the 

 central organ, the other towards the periphery. The centrally directed processes 

 soon reach and grow into the embryonic cord at its dorso-lateral aspect, where they 

 are presently seen in sections occupying an oval area near the periphery of the cord ; 



1 The place at which the anterior roots spring from the cord is not opposite to the corresponding 

 posterior root, but midway between that root and the succeeding one. 



