HEAD CAVITIES AND NERVES OF ELASMOBRANCHS., 895 
a nerve (v d., fig. 4) runs forward along the top of the second 
head cavity to the ciliary ganglion of the third nerve (c. g.), this 
being the communicating branch between the fifth and third 
nerves already mentioned. c. From the most ventrally situated 
part of the ganglion a nerve runs down in the interspace between 
the second and third cavities, and is then continued down 
in the mandibular arch; this may be called the mandibular 
nerve. 
Balfour describes only two branches of the fifth nerve at this 
stage, of which the posterior is the mandibular branch of the 
above description, whilst the anterior, which he describes and 
figures* as lying in close contact with the upper wall of the second 
cavity, and which he names the ophthalmic branch of the fifth, is 
clearly the second of the three branches I have described above. 
The first branch, to which, for reasons to be fully stated here- 
after, I prefer to give the name ophthalmic branch of the fifth, 
is not distinguished by Balfour from the second. 
By stage u certain changes have occurred. In the first place 
two or three new roots of origin are now present, which were not 
recognisable at the earlier date. ‘These roots, which are well 
shown in fig. 6 (vf), and also in fig. 11 (v/), are situated in 
front of the original roots of origin; they are very slender, are 
apparently variable in number, and differ materially from the 
original root by being totally devoid of ganglion cells. Another 
point of difference is that, opposite the ganglionic root of origin, 
the brain presents a distinct external prominence or bulging, 
well seen in figs. 6 and 11, while no such prominence occurs 
opposite these new roots. These additional roots increase in 
number during the later stages. By stage o one of them, usually 
the most anterior, has become considerably larger than the others 
(vide fig. 36, vf), though it still stands in marked contrast with 
the original root by having no ganglion cells, and by not pre- 
senting the external bulging of the brain at its point of origin. 
These roots will be again referred to later on. I have been unable 
to determine with certainty whether they arise as outgrowths 
from the brain or from the ganglion. 
By stage L an anterior branch—the maxillary nerve—is given 
off from the mandibular nerve. This branch is also described 
and figured by Balfour. 
Fig. 5, shows the roots of the ophthalmic and the communi- 
cating branches (v a. and v d.), and fig. 6 shows the ophthalmic 
branch (v a.), as well as the two kinds of roots of origin at 
stage L. Figs. 9 to 11 also show these branches in an embryo 
of the same age. 
Op. cit., p. 197. 
2 Op. cit., plate xiv, figs. 9 6 and 16 4, 
