77 2 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



found a vascular membrane enveloping the vitreous humour on 

 its retinal aspect, which, so far as we know, is unlike anything 

 which has so far been met with in the eye of any other adult 

 Vertebrate. 



The membrane itself is placed immediately outside the hya- 

 loid membrane, i.e. on the side of the hyaloid membrane bound- 

 ing the vitreous humour. It is easily removed from the retina, 

 to which it is only adherent at the entrance of the optic nerve. 

 In both the eyes we examined it also adhered, at one point, to 

 the capsule of the lens, but we could not make out whether this 

 adhesion was natural, or artificially produced by the coagulation 

 of a thin layer of albuminous matter. In one instance, at any 

 rate, the adhesion appeared firmer than could easily be produced 

 artificially. 



The arrangement of the vessels in the membrane is shewn 

 diagrammatically in Plate 38, fig. 49, while the characteristic 

 form of the capillary plexus is represented in Plate 38, fig. 50. 



The arterial supply appears to be derived from a vessel per- 

 forating the retina close to the optic nerve, and obviously homo- 

 logous with the artery of the processus falciformis and pecten 

 of Teleostei and Birds, and with the arteria centralis retinae of 

 Mammals. From this vessel branches diverge and pursue a 

 course towards the periphery. They give off numerous branches, 

 the blood from which enters a capillary plexus (Plate 38, figs. 

 49 and 50) and is collected again by veins, which pass outwards 

 and finally bend over and fall into (Plate 38, fig. 49) a circular 

 vein (cr. v.) placed at the outer edge of the retina along the 

 insertion of the iris (z>). 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 com- 

 posed of a transparent matrix, in which numerous cells are 

 embedded (Plate 38, fig. 50). 



Development. In the account of the first stages of develop- 

 ment of Lepidosteus, the mode of formation of the optic cup, the 

 lens, Sec., have been described (vide Plates 35 and 36, 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. 



