190 JOHN BEARD. 



possessing then what we may regard as its primitive branchial sense 

 organ. 



Later, the sensory thickening grows in an anterior direction, and as 

 it does so the ganglion separates from the skin, leaving behind it, as 

 in other cases, a nerve which is split off from the sensory thickening, 

 and which is the supra-branchial branch of the fifth (fig. 51, 02).s.). 

 Its course, &c, have been described by Marshall and Spencer, and it 

 is usually called the portio minor of the ophthal. superfic. It was first 

 classed as the r. dorsalis of the fifth by Balfour, and Marshall and 

 Spencer afterwards expressed their agreement with this view. Where 

 the main nerve fuses with the skin its course is continued along the 

 mandibular arch by a number of cells of the nerve. These form the 

 post-branchial branch, and innervate the musculature of the man- 

 dibular arch. Later, a pree-branchial nerve is developed (Van Wijhe 

 and others), which hooks over the angle of the mouth in the way that 

 other pree-branchial branches hook over gill-clefts. 



Another apparent branch of the fifth is the nerve which Marshall 

 has called a communicating nerve between the ciliary and Gasserian 

 ganglia (fig. 51, c.b.). Its true nature has been worked out by Van 

 Wijhe, who has shown that it really belongs to the ciliary ganglion. 

 As I accept this statement I shall describe the nerve, as Van Wijhe 

 has done, as part of the nerve of the second segment. 



The ophthalmicus profundus (fig. 51, oph.'pro.) is also a part of the 

 nerve of the second segment. This has been recognised by Marshall 

 and Spencer, and also by Van Wijhe. 



The later fusions which occur between the fifth and seventh and the 

 fifth and ciliary are in the early stages absent. In fact, in its develop- 

 ment the fifth has the triced characters of the posterior root of a gill- 

 hearing segment. It fulfils in every way, as Marshall found, the 

 requirements of a segmental nerve as laid down by him, and it accords 

 with our schema. It possesses a primitive branchial sense organ and 

 an associated ganglion just above a cleft, the mouth, It has the homo- 

 logues of post-branchial and pra>branchial branches, and it develops a 

 supra-branchial nerve in connection with the branchial sense organs 

 over the snout (fig. 51, ojxs.). 



The new additional light thrown on the nature of the mouth will be 

 referred to in discussing the general morphological considerations 

 arising out of these researches. Suffice it here to say that the facts 



