NERVOUS SYSTEM OF THE VERTEBRATA. 369 



(349) ^^- Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Sitz. d. Jemxi^chen 

 Gesell. f. Med. u. Naturwiss. Jan. 23, 1880. 



(350) Fried. Tiedemann. Anatomie und Bildimg><(feschichte des Gehirm im 

 Foetus des Memchen. Niiinberg, 1816. 



The development of the Cranial and Spinal Nerves \ 



All the nerves are outgrowths of the central nervous system, 

 but the differences in development between the cranial and spinal 

 nerves are sufficiently great to make it convenient to treat them 

 separately. 



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 outgrowths are the rudiments of the posterior 



1 Remak derived the posterior ganglia from the tissue of the mesoblastic somites, and 

 following in Remak's steps most authors believed the peripheral nervous system to have 

 a mesoblastic origin. This view, which had however been rejected on theoretical 

 grounds by Hensen and others, was finally attacked on the ground of observation by 

 His (No. 297). His (No. 352, p. 458) found that in the Fowl "the spinal ganglia of the 

 head and trunk arose from a small band of matter which is placed between the me- 

 dullary plate andepiblast, 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 inter- 

 mediate 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 ionv ganglia in the head, viz. the g. trigemini, g. acousticum, g. glosso- 

 pharyngei, 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 accoimt, 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 out-growths 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 re- 

 sult of his work his original statements. I cannot, however, accept his interpretations 

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

 His' own paper. 



B. E. II. 24 



