BRANCHIAL SENSE ORGANS OP ICHTHTOPSIDA. 193 



ing, cells are given off for some time, until a large ganglionic mass is 

 formed, which still for some time remains fused with the skin. 



In fact, in the case of the ciliary ganglion the mode of development 

 is well marked and very easy to study. The sensory thickening soon 

 begins to grow forwards over the snout, and as it does so the ganglion 

 begins to leave the skin. As this takes place a nerve is developed from 

 the thickening, and connects the ganglion with its branchial sense 

 organs. 



From its course, relations, &o, this nerve is seen to be the ophthal- 

 micus profundus. 1 It is morphologically the supra-branchial nerve of the 

 second segment. 



The distance between the ciliary and Gasserian ganglia, even in 

 early stages, is very short. The outgrowth from the neural ridge 

 which forms the main stem of the ciliary ganglion is practically con- 

 tinuous with the outgrowth which forms the main stem of the fifth. 

 Van Wljhe has also drawn attention to this. 



Hence it can hardly be wondered at that the connection of the two 

 ganglia with the brain soon becomes a common one, which distally 

 divides into two portions, one of which is continued to the Gasserian 

 ganglion, while the other goes somewhat obliquely to the ciliary, and 

 forms its so-called radix longa (fig. 51, c.h.). 



Although I have no observations to record as to the development of 

 the third or motor-oculi nerve, still Marshall's opinions on the nature 

 of the nerve must be discussed, and as his views are inconsistent with 

 the other facts as recorded in this paper. I shall state what seem to be 

 urgent reasons for modifying them. 



Marshall has advanced the suggestion that the third and fourth 

 nerves together make up a segmental nerve. He says: 2 "There is very 

 strong reason for thinking that, in the chick at any rate, the third 

 nerve develops, like the hinder cranial nerves and the posterior roots 

 of spinal nerves, as an outgrowth from the neural crest on the top of 

 the mid-brain." Since the third nerve later on arises from the base of 

 the mid-brain, "very near the mid-ventral line," he infers that the 

 nerve must shift downwards, and to an extent unequalled by any other 

 nerve. 



Now, leaving aside the fact that the shifting in the case of the third 



1 Apparently also Van Wijhe'a identification, but not very obvious from his description. 

 4 Marshall, " Segmental Value of Cranial Nerves," ' Journ. of Anat. and Physiol.,' p. 35 

 1882. 



