DEVELOPMENT OF ELASMOBRANCH FISHES. 197 



the point of attachment of the fifth nerv^e, it has not remained 

 in other respects in a stationary condition. 



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 mandi- 

 bular 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 situ- 

 ated 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 xiv. fig. 9 6 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. Be- 

 tween 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 ex- 

 tremity, and passing forwards, PL XIV. 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, PL xiv. 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 a single ophthalmic branch (PL XVI. fig. 41) Y op. th.), 

 which passes forwards slightly dorsal to the eye and parallel and 

 ventral to a branch of the seventh, which will be described when 

 I come to that nerve. I have been unable to observe that this 



