DEVELOPMENT OF THE ELASMOBRANCH FISHES. IO/ 



the second arch leaves hardly a doubt that it is equivalent to 

 one such nerve as those distributed to the posterior arches. 

 Subsequently it acquires another branch, passing forwards to- 

 wards the arch in front. 



The most anterior nerve is the fifth (fig. 14, v), of which 

 two branches are at this stage developed. The natural inter- 

 pretation of its present condition is, that it is equivalent to 

 two nerves, but the absence of relation in its branches to any 

 visceral clefts renders it more difficult to determine the mor- 

 phology of the fifth nerve than of the other nerves. The front 

 branch of the two is the ophthalmic branch of the adult, and the 

 hind branch the inferior maxillary branch. The latter branch 

 subsequently gives off low down, i.e. near its distal extremity, 

 another branch, the superior maxillary branch. 



In its embryonic condition this latter branch does not appear 

 like a third branch of the fifth, equivalent to the seventh or the 

 glosso-pharyngeal nerves, but rather resembles the branch of 

 the seventh nerve which passes to the arch in front, which also 

 is present in all the other cranial nerves. 



Modes of Preparation. 



Before concluding I will say one or two words as to my 

 modes of preparation. 



I have used picric and chromic acids, both applied in the 

 usual way ; but for the early stages I have found osmic acid by 

 far the most useful reagent. I placed the object to be hardened, 

 in osmic acid (half per cent.) for two hours and a half, and then 

 for twenty four in absolute alcohol. 



I then embedded and cut sections of it in the usual way, 

 without staining further. 



I found it advantageous to cut sections of these embryos 

 immediately after hardening, since if kept for long in the 

 absolute alcohol the osmic acid specimens are apt to become 

 brittle. 



