ORGANS OF VISION OF THE VERTEBRATA. 



503 



in Elasmobranchii is not included in the folding to which the secondary 

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

 optic cup only at the distal end of the processus falciformis. 



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 Schenk 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 falciformis 

 comes in contact with, and perhaps becomes 

 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- 

 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. 



FIG. 294. HORIZONTAL 

 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. 



In the Ammoccete 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 

 fcetal 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, 

 Kolliker, 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 

 the above embryologists, that 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 



