1/4 DEVELOPMENT OF THE SPINAL NERVES 



fig. D a, ar] as a small but distinct outgrowth from the epiblast 

 of the spinal cord, near the ventral corner of which it appears as 

 a conical projection. Even from the very first it has an indis- 

 tinct form of termination and a fibrous appearance, while the 

 protoplasm of which it is composed becomes very attenuated 

 towards its termination. 



The points of origin of the anterior roots from the spinal 

 cord are separated from each other by considerable intervals. 

 In this fact, and also in the nerves of the two sides never 

 being united with each other in the ventral median line, the 

 anterior roots exhibit a marked contrast to the posterior. 



There exists, then, in T^^Wo-embryos by the end of this 

 stage distinct rudiments of both the anterior and posterior 

 roots of the spinal nerves. These rudiments are at first quite 

 independent of and disconnected with each other, and both 

 take their rise as outgrowths of the epiblast of the neural 

 canal. 



The next Torpedo-embryo (PL 22, fig. D b), though taken 

 from the same female, is somewhat older than the one last 

 described. The cells of the notochord are considerably vacuo- 

 lated ; but the segmental duct is still without a lumen. The 

 posterior nerve-rudiments are elongated, pear-shaped bodies of 

 considerable size, and, growing in a ventral direction, have 

 reached a point nearly opposite the base of the neural canal. 

 They still remain attached to the top of the neural canal, 

 though the connexion has in each case become a pedicle so 

 narrow that it can only be observed with great difficulty. 



It is fairly certain that by this stage each posterior nerve- 

 rudiment has its own separate and independent junction with 

 the spinal cord ; their dorsal extremities are nevertheless pro- 

 bably connected with each other by a continuous commissure. 



The cells composing the rudiments are still round, and 

 have, in fact, undergone no important modifications since the 

 last stage. 



The important feature of the section figured (fig. D b), and 

 one which it shares with the other sections of the same embryo, 

 is the appearance of connective-tissue cells around the nerve- 

 rudiment. These cells arise from two sources ; one of these 

 is supplied by the vertebral rudiments, which at the end of 



