IN ELASMOBRANCH FISHES. 187 



are cut through as they are proceeding, in a more or less hori- 

 zontal course, from the spinal cord to the muscle-plates. 



Where the section (which is not quite horizontal) passes 

 through the plane of the notochord, as on the right-hand side, 

 the anterior roots are cut transversely. Each root, in fact, 

 changes its direction, and takes a downward course. 



The anterior roots are situated nearly opposite the middle 

 of the muscle-plates : their section is much smaller than that 

 of the posterior roots, and with haematoxylin they stain more 

 deeply than any of the other cells in the preparation. 



The anterior roots, so far as I have been able to observe, do 

 not at this stage unite with the posterior ; but on this point I do 

 not speak with any confidence. 



The period now arrived at forms a convenient break in the 

 development of the spinal nerves ; and I hope to treat the 

 remainder of the subject, especially the changes in the ganglion, 

 the development of the ganglion-cells, and of the nerve-fibres, 

 in a subsequent paper. 



I will only add that, not long after the stage last described, 

 the posterior root unites with the anterior root at a consider- 

 able distance below the cord : this is shewn in PI. 23, fig. L. 

 Still later the portion of the root between the ganglion and 

 the spinal cord becomes converted into nerve-fibres, and the 

 ganglion becomes still further removed from the cord, while at 

 the same time it appears distinctly divided into two parts. 



As regards the development of the cranial nerves, I have 

 made a few observations, which, though confessedly incomplete, 

 I would desire to mention here, because, imperfect as they are, 

 they "seem to shew that in Elasmobranch Fishes the cranial 

 nerves resemble the spinal nerves in arising as outgrowths from 

 the central nervous system. 



I have given a figure of the development of a posterior root 

 of a cranial nerve in fig. M I. The section is taken from the 

 same embryo as figs. B I, B II, and B III. 



It passes through the anterior portion of a thickening of 

 the external epiblast, which eventually becomes involuted as 

 the auditory vesicle. 



The posterior root of a nerve (VII) is seen growing out from 

 the summit of the hind brain in precisely the same manner that 



