1875.] Mr. F. M. Balfour on the Spinal Nerves in Fishes. 135 



I. " On the Development of the Spinal Nerves in Elasmobranch 

 Fishes." By F. M. Balfour, B.A., F.L.S., Fellow of Trinity 

 College, Cambridge. Communicated by Dr. Michael Foster, 

 F.H.S., Prselector in Physiology in Trinity College, Cam- 

 bridge. Received October 5, 1875. 



(Abstract.) 



The author commences by stating that the observations recorded in his 

 paper have been made upon the three following genera of Elasmobranchs 

 —Scyllium, Pristiurus, and Torpedo. 



The majority of his observations were made upon specimens procured 

 for him through the agency of the Zoological Station at Naples ; but he has 

 also been supplied in a most liberal manner with Scyllium embryos by 

 the Directors of the Brighton Aquarium, through the kindness of Mr. 

 Henry Lee. He finds that Torpedo embryos are by far more suitable than 

 any other genera he has employed for the investigation of the development 

 of the nerves. 



The author then gives a detailed account of his observations upon the 

 development of the spinal nerves in all the genera above mentioned. The 

 following are the chief results at which he has arrived. 



The posterior and anterior roots of the spinal nerves arise as inde- 

 pendent outgrowths from the involuted epiblast of the neural canal. 



The outgrowths for the two roots are at first quite independent of each 

 other, and only unite at a late period of development. 



The posterior roots are the first to develop. An outgrowth arises on 

 each side from the dorsal summit of the neural canal, which the author 

 believes to be unbroken throughout its whole length. The outgrowths 

 on the two sides are at first in contact with each other; and from each 

 there springs a series of processes equal in number to the muscle-plates. 

 t These processes are the rudiments of the posterior nerve-roots. They 

 grow ventral wards in contact with the side of the spinal cord. 



After the formation of the posterior rudiments, the original outgrowths 

 from the spinal cord cease to be attached to it along its whole length, and 

 remain in connexion with it at a series of points only, each of which cor- 

 responds to a posterior root. 



The result of these changes is the formation of a series of nerve-roots, 

 each attached to the dorsal summit of the neural canal, and all of them 

 united together dorsally by a continuous commissure, which is the rem- 

 nant of the primitive outgrowth from the summit of the neural canal. 



Subsequently the points of attachment of the posterior roots travel 

 down the sides of the spinal cord, and finally remain fixed at about one 

 third of the distance from its dorsal summit. At the same time the nerve- 

 rudiments undergo histological changes, by which each becomes divided 

 into a root, a ganglion, and a nerve. 



