176 RICHARD ASSHETON, M.A. 



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 the opposite 

 direction to what it does in Vertebrate eyes of the present day. 



That is to say, the light-perceiving portion of the sensory cell would 

 be directed towards the cavity of the brain, and the transmitting 

 portion or nerve-fibre towards the exterior, as indeed is the case in the 

 larval Ascidian eye. 



When we realise that the nerve-fibres of the present Vertebrate eye 

 really pass over the edge of the cup, and do not — morphologically 

 speaking — pierce it, we are able to imagine the probable steps in the 

 evolution of the eye far more easily than otherwise. 



It can hardly be supposed that such eyes perceived any image ; it 

 was merely a case of distinguishing light from dark. On the closure 

 of the neural tube in the one case, or on the commencement of opacity 

 in the other, and more light therefore reaching the light-sensitive cells 

 from the opposite direction to that heretofore, any variation (1) which 

 brought the sensory patch nearer the skin (origin of optic vesicle), (2) 

 which brought the skin nearer the sensory patch, i.e., depression in 

 the skin (origin of lens), would tend to be preserved. 



As yet the eye would not be a cup ; it would only become so in 

 connection with the formation of the lens. 



Round the depression in the skin the light sensitive area might 

 expand, and by the growth of its edges round the depression in the 

 skin would form a cup. 



While these changes were in progress the nerve-fibres having now 

 to pass over one part of the edge of the area to reach their cerebral 

 destination, would prevent the growth of the edge at that point, and 

 consequently a gap would be left. 



As soon as a lens was formed and an image thrown upon the 

 retina, a gap would be disadvantageous to the perception of the image 

 as well as to the retention of the vitreous body, which no doubt 

 existed as early as the lens ; but until such a time I do not see why 

 a choroid fissure should not be a permanent feature ; and indeed it 



