420 DEVELOPMENT OF ELASMOBRANCH FISHES. 



that they spring from the brain about half way up its sides, and 

 a little ventral to the border of its thin roof. While this change 

 has been taking place in the point of attachment of the fifth 

 nerve, it has not remained in other respects in a stationary con- 

 dition. 



During stage H it already exhibits two distinct branches 

 known as the mandibular and ophthalmic. These branches first 

 lie outside a section of the body cavity which exists in the front 

 part of the head. The ophthalmic branch of the fifth being 

 situated near the anterior end of this, and the mandibular near 

 the posterior end. 



In stage I the body cavity in this part becomes divided into 

 two parts one behind the other, the posterior being situated in 

 the mandibular arch. The bifurcation of the nerve then takes 

 place over the summit of the posterior of the two divisions of 

 the body cavity, PL 15, figs. 9 b, V. and 10, V, &c., and at first 

 both branches keep close to the sides of this. 



The anterior or ophthalmic branch of the fifth soon leaves the 

 walls of the cavity just spoken of and tends towards the eye, 

 and there comes in close contact with the most anterior section 

 of the body cavity which exists in the head. These relations it 

 retains unchanged till the close of stage K. Between stages I 

 and K it may easily be seen from the surface ; but, before the 

 close of stage K, the increased density of the tissues renders it 

 invisible in the living embryo. 



The posterior branch of the fifth extends downwards into the 

 mandibular arch in close contact with the posterior and outer 

 wall of the body space already alluded to. At first no branches 

 from it can be seen, but I have detected by the close of stage K, 

 by an examination of the living embryo, a branch springing 

 from it a short way from its central extremity, and passing for- 

 wards, PI. 15, fig. 2, V. This branch I take to be the rudiment 

 of the superior maxillary division of the fifth nerve. It is shewn 

 in section, PI. 15, fig. 15 a, V. 



In the stages after K the anatomy of the nerves becomes 

 increasingly difficult to follow, and accordingly I must plead 

 indulgence for the imperfections in my observations on all the 

 nerves subsequently to this date. In the fifth I find up to 

 stage O a single ophthalmic branch (PI. 17, fig. 4 b, V op. th.}, 



