NERVOUS SYSTEM OF THE VERTEBRATA. 449 



Spinal nerves. The posterior roots of the spinal nerves, as 

 well as certain of the cranial nerves, arise in the same manner, 

 and from the same structure, and are formed considerably before 

 the anterior roots. Elasmobranch fishes may be taken as the 

 type to illustrate the mode of formation of the spinal nerves. 



The whole of the nerves in question arise as outgrowths of a 

 median ridge of cells, which makes its appearance on the dorsal 

 side of the spinal cord (fig. 264 A, pr). This ridge has been 

 called by Marshall the neural crest. At each point, where a 

 pair of nerves will be formed, two pear-shaped outgrowths 

 project from it, one on each side ; and apply themselves closely 

 to the walls of the spinal cord (fig. 264 B, pr). These out- 

 growths are the rudiments of the posterior nerves. While still 

 remaining attached to the dorsal summit of the neural cord they 

 grow to a considerable size (fig. 264 B, pr). 



The attachment to the dorsal summit is not permanent, but 



spinal ganglia of the head and trunk arose from a small band of matter which is 

 placed between the medullary plate and epiblast, and the material of which he called 

 the 'intermediate cord'." He further states that: "Before the closure of the 

 medullary tube this band forms a special groove the 'intermediate groove' placed 

 close to the border of the medullary plate. As the closure of the medullary plate 

 into a tube is completed, the earlier intermediate groove becomes a compact cord. 

 In the head of the embryo a longitudinal ridge arises in this way, which separates the 

 suture of the brain from that of the epiblast. In the parts of the neck and in the 

 remaining region of the neck the intermediate cord does not lie over the line of 

 junction of the medullary tube, but laterally from this and forms a ridge, triangular 

 in section, with a slight indrawing." This intermediate ridge gives rise to four 

 ganglia in the head, viz. the g. trigemini, g. acousticum, g. glossopharyngei, and 

 g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the 

 spinal cord. 



I have given in the above account, as far as possible, a literal translation of His' 

 own words, because the reader will thus be enabled fairly to appreciate his meaning. 



Subsequently to His' memoir (No. 297) I gave an account of some researches of 

 my own on this subject (No. 351), stating the whole of the nerves to be formed as 

 cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the 

 stages I spoke of had been already accurately described by His, though interpreted by 

 him very differently. Marshall, and afterwards Kolliker, arrived at results in the main 

 similar to my own, and Hensen, independently of and nearly simultaneously with 

 myself, published briefly some observations on the nerves of Mammals in harmony 

 with my results. 



His has since worked over the subject again (No. 352), and has reaffirmed as a 

 result of his work his original statements. I cannot, however, accept his interpreta- 

 tions on the subject, and must refer the reader who is anxious to study them more 

 fully, to His' own paper. 



B. III. 29 



