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MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
periphery. They give off numerous branches, the blood from which enters a capillary 
plexus (Plate 25, figs. 49 and 50) and is collected again by veins, which pass outwards 
and finally bend over and fall into (Plate 25, fig. 49) a circular vein (cr.v.) placed at the 
outer edge of the retina along the insertion of the iris (ir). The terminal branches of 
some of the main arteries appear also to fall directly into this vein. 
The membrane supporting the vessels just described is composed of a transparent 
matrix, in which numerous cells are embedded (Plate 25, fig. 50). 
Development —In the account of the first stages of development of Lepidosteus, the 
mode of formation of the optic cup, the lens, &c., have been described (vide Plates 22 and 
23, figs. 23, 26, 35). With reference to the later stages in the development of the eye, 
the only subject with which we propose to deal is the growth of the mesoblastic 
processes which enter the cavity of the vitreous humour through the choroid slit. 
Lepidosteus is very remarkable for the great number of mesoblast cells which thus 
enter the cavity of the vitreous humour, and for the fact that these cells are at first 
unaccompanied by any vascular structures (Plate 24, fig. 43, v.h.). The mesoblast 
cells are scattered through the vitreous humour, and there can be no doubt that 
during early larval life, at a period however when the larva is certainly able to see, 
every histologist would consider the vitreous humour to be a tissue formed of scattered 
cells, with a large amount of intercellular substance ; and the fact that it is so appears 
to us to demonstrate that Kessler’s view of the vitreous humour being a mere 
transudation is not tenable. 
In the larva five or six days after hatching, and about 15 millims. in length, the 
choroid slit is open for its whole length. The edges of the slit near the lens are 
folded, so as to form a ridge projecting into the cavity of the vitreous humour, while 
nearer the insertion of the optic nerve they cease to exhibit any such structure. The 
mesoblast, though it projects between the lips of the ridge near the lens, only extends 
through the choroid slit into the cavity of the vitreous humour in the neighbourhood 
of the optic nerve. Here it forms a lamina with a thickened edge, from which scattered 
cells in the cavity of the vitreous humour seem to radiate. 
At a slightly later stage than that just described, blood-vessels become developed 
within the cavity of the vitreous humour, and form the vascular membrane already 
« 
described in the adult, placed close to the layer of nerve-fibres of the retina, but separated 
from this layer by the hyaloid membrane (Plate 25, fig. 48, v.sh.). The artery bring¬ 
ing the blood to the above vascular membrane is bound up in the same sheath as the 
optic nerve, and passes through the choroid slit very close to the optic nerve. Its 
entrance into the cavity of the vitreous humour is shown in Plate 25, fig. 48 (vs .); its 
relation to the optic nerve in Plate 24, fig. 46, C and D (vs.). 
The above sheath has, so far as we know, its nearest analogue in the eye of Alytes, 
where, however, it is only found in the larva. 
The reader who will take the trouble to refer to the account of the imperfectly- 
developed processus falciformis of the Elasmobmnch eye in the treatise ‘On Comparative 
