STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 773 



Lepidosteus is very remarkable for the great number of meso- 

 blast 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 37, 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 con- 

 sider 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 ex- 

 hibit 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 

 38, fig. 48, v.s/1.). The artery bringing 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 

 shewn in Plate 38, fig. 48 (vs.); its relation to the optic nerve in 

 Plate 37, 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 falcifprmis of the Elas- 



