118 JOHN BEARD. 
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 deve- 
lopment of the supra-branchial nerve (portio minor of the 
ophthal. 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 rudi- 
ment of the fifth, is broad and extends backwards almost to 
the region of the seventh. Anteriorly it stretches forwards 
almost to the region 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, A. ¢. 2). 
The nerve rudiment grows down to the level of the noto- 
chord (fig. 14), and fuses with an epiblastic thickening, just as 
the other nerves do. Here cells can be seen leaving the thick- 
ening 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 
dorsad of 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 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, op.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. dor- 
salis of the fifth by Balfour, and Marshall and Spencer after- 
wards expressed their agreement with this view. Where the 
