100 EICHAUD ASSHETON. 



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 Verte- 

 brate 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 there- 

 fore reaching the light-sensitive cells from the opposite direc- 

 tion 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. depres- 

 sion 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 per- 

 manent feature; and indeed it seems to me that a conside- 

 ration of the manner in which the Vertebrate eye was evolved 

 almost necessitates the occurrence of a gap at a certain stage. 



