BRANCHIAL SENSE ORGANS OF ICHTHYOPSIDA. 189 



ganglion, which in Elasmobranchs, and even in birds, is qxiite distinct 

 in its development, is in the Amphibians fused with the Gasserian, and 

 the two arise together as one fused mass. 



Vagus 1, 2, 3, and 4 are also all fused into one mass in Amphibia; 

 the figure (27) is a transverse section through this mass. In it the 

 nerve has not separated from the skin, and the ganglionic portion is 

 readily recognisable as a mass of yolk-filled cells on the level of the 

 lateral line. Later, both ganglion and nerve leave the skin as in 

 Elasmobranchs. 



Nerve of the Third Segment — Trigeminal less Ophthalmicus 



Profundus. 



The fifth nerve is well suited for studying the development of the 

 ganglion of a dorsal root. 



It is well known, from Balfour's and Marshall's researches (opera 

 cit), that it arises from the thud of the brain vesicles. In fact, from 

 their researches and those of Van Wijhe, the development of the fifth 

 is fairly well known with the exception of three stages. These are the 

 fusion with the skin, the formation of the Gasserian ganglion, and the 

 mode of development of the supra-branchial nerve (portio minor of the 

 ophthalmicus superficialis, Schwalbe). 



To explain these stages it will be necessary to repeat some facts 

 which are already known. 



The outgrowth from the neural ridge, which forms the rudiment of 

 the fifth, is broad and extends backwards almost to the region of the 

 seventh. Anteriorly it stretches forwards almost to the roots of the 

 ciliary, to be hereafter mentioned. 



But the region between the two ganglia is well defined in the earliest 

 stages by the indifferent epithelium between them, and by the position 

 of the second head cavity which lies between them (fig. 11, h.c. 2). 



The nerve rudiment grows down to the level of the notochord (fig. 14) 

 and fuses with an epiblastic thickening, just as the other nerves do. 

 Here cells can be seen leaving the thickening to form the ganglion 

 (fig. 15). 



In this case and in that of the ciliary there can be little doubt as to 

 the actual mode of formation of the ganglion. The thickening which 

 gives rise to the ganglion is situated just dorsal to the mouth, and in 

 fact has just the position of a branchial sense organ. 



The ganglion is figured in fig. 17, still connected with the skin, and 



