ORGANS OF VISION. 509 



This explanation may possibly be correct, but another explanation appears 

 to me possible, and I am inclined to think that the vertebrate eyes have not 

 been derived from eyes like those of Ascidians, but that the latter is a 

 degenerate form of vertebrate eye. 



The fact of the retina being derived from the fore-brain may perhaps be 

 explained in the same way as has already been attempted in the case of the 

 retina of the Crustacea ; i.e. by supposing that the eye was evolved simulta- 

 neously with the fore part of the brain. 



The peculiar processes which occur in the formation of the optic vesicle 

 are more difficult to elucidate ; and I can only suggest that the development 

 of a primary optic vesicle, and its conversion into an optic cup, is due to the 

 retinal part of the eye having been involved in the infolding which gave rise 

 to the canal of the central nervous system. The position of the rods and 

 cones on the posterior side of the retina is satisfactorily explained by this 

 hypothesis, because, as may be easily seen from figure 285, the posterior face 

 of the retina is the original external surface of the epidermis, which is 

 infolded in the formation of the brain ; so that the rods and cones are, as 

 might be anticipated, situated on what is morphologically the external surface 

 of the epiblast of the retina. 



The difficulty of this view arises in attempting to make out how the eye 

 can have continued to be employed during the gradual change of position 

 which the retina must have undergone in being infolded with the brain in 

 the manner suggested. If however the successive steps in this infolding 

 were sufficiently small, it seems to me not impossible that the eye might have 

 continued to be used throughout the whole period of change, and a trans- 

 parency of the tissues, such as Lankester suggests, may have assisted in 

 rendering this possible. 



The difficulty of the eye continuing to be in use when undergoing 

 striking changes in form is also involved in Lankester's view, in that if, as I 

 suppose, he starts from the eye of the Ascidian Tadpole with its lenses 

 turned towards the cavity of the brain ; it is necessary for him to admit that 

 a fresh lens and other optical parts of the eye became developed on the 

 opposite side of the eye to the original lens ; and it is difficult to understand 

 such a change, unless we can believe that the refractive media on the two 

 sides were in operation simultaneously. It may be noted that the same 

 difficulty is involved in supposing, as I have done, that the eye of the 

 Ascidian Tadpole was developed from that of a Vertebrate. I should 

 however be inclined to suggest that the eye had in this case ceased for a 

 period to be employed ; and that it has been re-developed again in some of 

 the larval forms. Its characters in the Tunicata are by no means constant. 



Accessory eyes in the Vertebrata. 



In addition to the paired eyes of the Vertebrata certain organs are 

 found in the skin of a few Teleostei living in very deep water, which, though 

 clearly not organs of true vision, yet present characters which indicate that 



