414 



THE CHOROID FISSURE. 



the walls of the optic cup only at the distal end of the processtis falci- 

 formis. 



In Teleostei there is at first a vascular loop like that in Birds, 

 passing through the choroid fissure. This has been noticed by Kessler in 

 the Pike, and by Scheiik in the Trout. At a later period a mesoblastic 

 ingrowth with a blood-vessel makes its way in many forms into the cavity 

 of the vitreous humour, accompanied by two folds in the walls of the free 

 edges of the choroid fissure (fig. 294). These structures, which constitute 



the processus falciformis, clearly resemble very 

 closely the mesoblastic process and folds of the 

 optic cup in Elasmobranchii. The processus fal- 

 ciformis comes in contact with, and perhaps be- 

 comes attached to the wall of the lens ; and 

 persists through life. 



In Triton there is no vascular ingrowth 

 through the choroid fissure, but a few meso- 

 blastic cells pass in which represent the vascular 

 ingrowth of other types. The optic nerve per- 



HoKI7ONTAI 



forates the proximal extremity of the original 

 choroid slit. 



The absence of an embryonic blood-vessel 

 does not however hold good for all Amphibia, 

 as there is present in the embryo Alytes (Lieber- 

 kiihn) an artery, which breaks up into a capillary 

 system on the retinal border of the vitreous 

 humour. 



In the Ammocoete the choroid slit is merely 

 represented by a slight notch on the ventral 



edge of the optic cup, and the mesoblastic process which passes through 

 the choroid slit in most types is represented by a large cellular process, 

 from which the vitreous humour would appear to be derived. 



Mammalia differ from all the types already described in the immense 

 foetal development of the blood-vessels of the vitreous humour. There are 

 however some points in connection with the development of these vessels 

 which are still uncertain. The most important of these points concerns 

 the presence of a prolongation of the mesoblast around the eye into the 

 cavity of the vitreous humour. It is maintained by Lieberkiihn, Arnold, 

 Kiilliker, etc., that in the invagination of the lens a thin layer of mesoblast 

 is carried before it ; and is thus transported into the cavity of the vitreous 

 humour. This is denied by Kessler, but the layer is so clearly figured by 

 tlfe above embryologists, tliat the existence of it in some Mammalia (the 

 Rabbit, etc.) must I think be accepted. 



In the folding in of the optic vesicle, which accompanies the formation 

 of the lens, the optic nerve becomes included, and on the development of 

 the cavity of the vitreous humour an artery, running in the fold of the 

 optic nerve, passes through the choroid slit into the cavity of the vitreous 

 humour (fig. 295 acr]. The sides of the optic nerve subsequently bend 

 over, and completely envelope this artery, which at a later period gives off 

 branches to the retina, and becomes known as the arteria centralis 

 retinae. It is homologous with the arterial limb of the vascular loop 

 projecting into the vitreous humour in Birds, Lizards, Teleostei, etc. 



Before becoming enveloped in the optic nerve this artery is continued 



FIG. 294. 

 SECTION THROUGH THE EYE OF 

 A TELEOSTEAN EMBRYO. (From 

 Gegenbaur; after Schenk.) 



s. choroid fissure, with 

 two folds forming part of the 

 processus falciformis ; a. cho- 

 roid layer of optic cup ; b. 

 retinal layer of optic cup ; c. 

 cavity of vitreous humour ; d. 

 lens. 



