ORGANS OF VISION OF THE YERTEBRATA. 413 



In the Lizard the choroid slit closes considerably earlier than in the 

 Fowl. The vascular loop in the vitreovis humour is however more de- 

 veloped. Tlie pecten long remains without vessels, and does not in fact 

 become at all vascular till after the very late disappearance of the distal 

 part of the vascular loop of the vitreous humour. 



The arrangement of the ingrowth through the choroid slit in Elasmo- 

 branchii (Scyllium) has been partially worked out, and so far as is at pi-esent 

 known the agreement between the Avian and Elasmobranch type is fairly 

 close. 



At the time when the cavity betw-een the lens and the secondary optic 

 cup is just commencing to be formed, a process of mesoblast accompanied 

 by a vascular loop passes into the vitreous humour, through the choroid 

 slit, close to the optic nerve. The vessel in this process is no doubt 

 equivalent to the vascular loop in the Avian eye, but I have not made 

 out that it projects beyond the mesoblastic process accompanying it. As 

 the cavity of the vitreous humour enlarges and the choroid slit elongates, 

 the process through it takes the form of a lamina with a somewhat swollen 

 border, and projects for some distance into the cavity of the vitreous 

 humour. 



At a later stage, after the outer layer of the ojitic cup has become pig- 

 mented, the distal part of the choroid slit adjoining the border of the lens 

 closes up ; but along the line where it was present the walls of the optic 

 cup remain very thin and are thrown into three folds, two lateral and 

 one median, projecting into the cavity of the vitreous humour. The 

 median fold is in contact with the lens, and the vascular mesoblast sur- 

 rounding the eye projects into the space between the two laminae of 

 which it is formed. In passing from the region of the lens to that of the 

 optic nerve the lateral folds of the optic cup disappear, and the median 

 fold forms a considnrable projection into the cavity of the vitreous humour. 

 It consists of a core of mesoblast covered by a delicate layer derived from 

 both strata of the optic cup. Still nearer the optic nerve the choroid slit 

 is no longer closed, and tlie mesoblast, which in the neighbourhood of the 

 lens only extended into the folds of the wall of the optic cup, now ])ro- 

 jects freely into the cavity of the vitreous humour, and forms the lamina 

 ali-eady described. It is not very vascular, but close to the optic nerve 

 there passes into it a considerable artery. 



In the young animal the choroid slit is no longer perforated by a meso- 

 blastic lamina. At its inner end it remains open to allow of the passage 

 of the optic nerve. The line of the slit can easily be traced along the 

 lower side of the retina ; and close to the lens the retinal wall continues, 

 as in the embryo, to be raised into a projecting fold. Traces of these 

 structures are visible even in the fully grown examples of Scyllium. 



As has been pointed out by Bergraeister the mesoblastic lamina pro- 

 jecting into the vitreous humour resembles the pecten at an early stage of 

 development, and is without doubt homologous with it. Tlie artery which 

 supplies it is certainly equivalent to the artery of the pecten. 



There can be no doubt that the mesoblastic lamina projecting into the 

 vitreous humour is equivalent to the processus falciformis of Teleostei, and 

 it seems probable that the whole of it, including the free part as well 

 as that covered by epiblast, ought to be spoken of under this title. The 

 optic nerve in Elasmobranchii is not included in the folding to which 

 the secondary optic vesicle owes its origin, and would seem to perforate 



