DEVELOPMENT OF THE OPTIC NERVE OF VERTEBRATES. 175 



So also, if the other elements of the eye be traced through the 

 section, it can be noticed that as they near the rim they are less 

 and less differentiated, until they all merge into a mass of rapidly 

 dividing cells, each one very like to his neighbour. 



The Choroidal Fissure. 



At one point of the rim, however, there is no proliferation of cells, 

 and therefore at this point the wall of the optic cup does not grow. 

 This point is that over which the fibres from the retinal neuroblasts 

 pass on their way towards the brain (fig. 12, CH). 



It is quite inaccurate to talk of the fibres of the optic nerve 

 becoming connected with the elements of the inner or retinal wall of 

 the cup after piercing the outer wall of pigment epithelium (Foster, G), 

 as the development shows that the fibres never really pierce either 

 wall, but, from the moment of their first formation, they are on the 

 outside of both. It is only by the subsequent growth of the rim of 

 the optic cup that the bundle of nerve-fibres becomes surrounded by 

 the walls of the cup, and so apparently pierces it. It does in reality 

 pass over the edge of the cup, just as much as do the fibres in such an 

 eye as that of Pecten. 



It has been usual to regard the choroidal fissure as essentially an 

 embryonic feature, present chiefly for the purpose of admitting 

 mesoblastic tissues into the optic cup for the formation of and nourish- 

 ment of the vitreous body, and to be due to the manner of invagination 

 of the optic vesicle. 



Some authors have recognised a further meaning in that the optic 

 nerve is thereby brought into connection with the retina (v. Hertwig, 

 9, p. 404). 



I have never, however, seen it suggested that the choroidal fissure 

 represents a stage in the evolution of the eye, as seems to me more 

 than probable, and that it was due entirely to the eye having a deep- 

 seated cerebral origin, and having only subsequently grown towards 

 the surface. 



Whatever may have been the first origin of the eyes of Vertebrates, 

 whether they arose, as has been suggested by Balfour (1), as patches 

 of the epidermis sensitive to light, before the sinking down and folding 

 up of the central nervous system, or whether, as Lankester (15, 16) 



