192 JOHN BEARD. 



Taking the ninth nerve, or glossopharyngeal, as a type of a cranial 

 nerve to a true gill-cleft, we see that there is a mam stern (p.r.), a 

 ganglion with associated sense organ, and then three other branches. 

 These are a post-branchial (p.m.), a prge-branchial (p.b.n.), and a 

 supra-branchial (s.b.?i.). As their names imply, the post-branchial 

 and prae-branchial run behind and in front of the cleft respectively. 

 The supra-branchial nerve is the nerve connected with the later 

 developed additional branchial sense organs. 



Now we may turn to the nerve of the second segment. The first 

 thing noticeable is that the cleft is absent, 1 or at any rate the gill 

 muscles are not present even in the ontogeny. 



As a natural corollary to the absence or metamorphosis of the cleft, 

 and absence of its muscles, the post-branchial and prce-branchial nerves 

 are also aborted. 



In the diagram this abortion is represented by dotted lines (fig. 46). 

 Hence all that we can expect to find of the posterior root of this 

 segment is a supra-branchial branch to the branchial sense organs, the 

 ganglion cf the branchial sense organs, and the main stem connecting 

 the ganglion with the brain. The ganglion is the ciliary, the main 

 stem is the radix longa, connecting the ciliary and Gasserian ganglia, 

 and the supra-branchial branch is the ophthalmicus profundus. 



This identification is very similar to that given by Van Wijhe, but 

 the matter is approached from an entirely different point of view. 



The actual development is as follows : From the neural crest of the 

 mid-brain, just before the closure of the neural folds, cells grow out- 

 wards and downwards to a thickened patch of epiblast just above and 

 behind the eye (fig. 7). 



This outgrowth has been seen and described by Marshall and Van 

 Wijhe. But Marshall recognised in it the first rudiment of the motor- 

 oculi, and Van Wijhe that of the ophthalmicus profundus. Neither 

 observer saw the skin fusion or the development of the ganglion. 

 When the outgrowth reaches the thickened patch of epiblast it fuses 

 with it (fig. 6). Cells are then proliferated off from the skin to form 

 the ganglion, and the outer portion of the thickening begins to form 

 the primitive branchial sense organ (figs. S and 9). From the thicken- 



1 Or metamorphosed. Dohrn has recognised what he believes to be a cleft behind the 

 nose and in front of the mouth in the hypophysis. He does not say that it is the cleft of the 

 ciliary ganglion, but this would seem to follow if Dohrn's view were accepted. As at 

 present, though possible, no relationship of this supposed cleft to the ciliary ganglion has 

 yet been demonstrated, Dohrn's yiew must be accepted with reserve. 



