502 THE CHOROID FISSURE. 



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 present 

 known the agreement between the Avian and Elasmobranch type is fairly 

 close. 



At the time when the cavity between 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 pro- 

 jects 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 optic 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 surrounding 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 considerable 

 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 

 the mesoblast, which in the neighbourhood of the lens only extended into the 

 folds of the wall of the optic cup, now projects freely into the cavity of the 

 vitreous humour, and forms the lamina already 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 Bergmeister 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. The 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 



