IN ELASMOBRANCH FISHES. 185 



msnt are far looser than in the last stage, and are commencing 

 to throw out processes and become branched. 



The anterior root-nerve has grown very considerable since 

 the last stage. It projects from the same region of the cord as 

 before, but on approaching the muscle-plate takes a sudden 

 bend downwards (fig. H II, ar). 



I have failed to prove that the anterior and posterior roots 

 are at this stage united. 



Fourth Stage. 



In an embryo but slightly more advanced than the one last 

 described, important steps have been made in the development 

 of the nerve-rudiment. The spinal cord itself now possesses a 

 covering of white matter; this is thickest at the ventral portion 

 of the cord, and extends to the region of the posterior root of 

 the spinal nerve. 



The junction of the posterior root with the spinal cord is 

 easier to observe than in the last stage. 



It is still effected by means of unaltered cells, though the 

 cells which form the projection from the cord to the nerve are 

 commencing to undergo changes similar to those of the cells 

 which are being converted into white matter. 



In the rudiment of the posterior root itself there are still 

 three distinct parts, though their arrangement has undergone 

 some alteration and their distinctness has become more marked 

 (PL 23, fig. I I). 



The root of the nerve (fig. 1 1, pr) consists, as before, of nearly 

 circular cells, each containing a nucleus, very large in propor- 

 tion to the size of the cell. The cells have a diameter of about 

 3oW f an mcn - This mass forms not only the junction 

 between the ganglion and the spinal canal, but is also con- 

 tinued into a layer investing the outer side of the ganglion and 

 continuous with the nerve beyond the ganglion. 



The cells which compose the ganglion (fig. I I, sp. g] are 

 easily distinguished from those of the root. Each cell is elon- 

 gated with an oval nucleus, large in proportion to the cell ; and 

 its protoplasm appears to be continued into an angular, not 

 to say fibrous process, sometimes at one and more rarely at 



