DEVELOPMENT OF THE OPTIO NEEVE OP VEETEBEATES. 99 



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 epi- 

 thelium (Foster, 6), 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 essen- 

 tially an embryonic feature, present chiefly for the purpose of 

 admitting mesoblastic tissues into the optic cup for the forma- 

 tion of and nourishment 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) suggested, they are 

 derived from such an eye as that found within the cerebral 

 vesicles of certain Ascidians, it is clear that they were of 

 myelonic origin, and much more deeply placed than at present 

 in adult Vertebrates. 



In either case the light must have fallen directly upon the 

 sensitive cell; that is to say, the light reached the eye from 



