768 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 
ganglionic layer is composed of large cells, which form a remarkably broad layer—quite 
unlike the narrow ganglionic stratum in the Salmonide. Anteriorly it is defined by the 
fibres of the optic nerve, and the limitans interna (Horrman’s “limitans externa”) or 
anterior limiting membrane, which forms the lining of the optic globe. Some observers 
look upon this membrane as the hyaloid capsule of the vitreous humour; HUvLKE, 
however, regards it as a separate membrane, and such it would appear to be, since it 
precedes the formation of the vitreous fluid by a long interval (wide No. 71, p. 248). 
An anterior annulus, the lip of the secondary optic vesicle or cup, remains unaffected 
by these histological changes, and a mass of indifferent cells fills up the interspace between 
the retina proper and the circular curtain—the extension of the choroid in front of the 
eye. ‘These cells are in fact involved in the formation of the iris and ciliary ridges, the 
ciliary muscles being developed from the mesoblast (mes) entering by the choroidal fissure 
(Pl. IV. figs. 19, 20). Even in the later larval stages this complex anterior annulus, 
formed of the cells just mentioned, and the pigmented choroid which grows round to 
enclose a circular opening in front of the eye—the pupil, constitutes a brilliantly 
opalescent iris, which adds to the remarkable appearance of the minute transparent 
larva (Pl. XVI. figs. 3, 7,9; Pl. XVIII. fig. 11). 
Cranial Nerves.—The optic, olfactory, and auditory nerves are treated elsewhere, and 
in this place only the larger and more important nerve-origins will be referred to, the 
Teleostean embryo being little favourable for tracing the development of the smaller 
cranial nerves, such as III., IV., and VI. The trigeminal (V.) is large, and readily made 
out. In Elasmobranchs it arises as two lateral outgrowths from a median dorsal ridge at 
the anterior end of the hind-brain. Ata late embryonic stage this nerve springs from 
the upper lateral margin of the hind-brain, but so far forward that the optic lobe covers 
it at this point, and it appears to emerge from the overlapping lobes at a point imme- 
diately posterior to the eyes. This lateral position must be secondary (as MarsHALL 
suggests in the case of Scyllium), the original median dorsal position being altered by the 
rapid growth of the roof of the brain, so that the origins of this pair of nerves become 
further and further separated, until finally they are lateral (No. 101). Just as the 
nerve emerges it separates into several rami, the outermost being the maxillo-palatine 
branch, while a second large branch, the mandibular, passes backward a short distance in 
close contact with the side of the medulla oblongata. Each of these main rami shows, near 
its origin, a very large ganglion, the two ganglia being so close together as to appear 
like slightly separated moieties of one primary ganglionic swelling. From the ganglion of 
the maxillary nerve a small nervous branch passes forward over the orbital arch, possibly 
the abducens (VI.), though more probably it is the ramus ophthalmicus of the VIth nerve. 
Between the two main rami just mentioned a large blood-vessel passes, and a third 
ganglion appears beneath it, also apparently one of the trigeminal group, while a slender 
nerve, whose destination could not be made out, was connected with this smaller ganglion. 
A little posterior to the trigeminal the VIIth and VIIIth arise in close proximity to each 
other, the auditory being posterior and exhibiting a large ganglion. 
