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 Salmonidse. Anteriorly it is defined by the 

 fibres of the optic nerve, and the limitans interna (Hoffman'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 ; Hulke, 

 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 (vide 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 

 (PI. 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 (PI. 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. At a 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 Vlth 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 Vllth and Vlllth arise in close proximity to each 

 other, the auditory being posterior and exhibiting a large ganglion. 



