11AXS 01'' \'ISIOX. 



General considerations on the Eye of the 



There can he but little doubt that the eye of the Tunicata belongs to 

 the same phylum as that of the true Vertebrata, different as the two eyes 

 are. The same may also be said with reference to the degenerate and very 

 rudimentary eye of Ampliioxus. 



The peculiarity of the eye of all the Chordata consists in the retina 

 being developed from part of the wall of the brain. How is this remark- 

 able feature of the eye of the Chordata to be explained ? 



Lankester, interpreting the eye in the light of the Tunicata, has made 

 the interesting suggestion 1 " that the original Vertebrate must have been a 

 transparent animal, and had an eye or pair of eyes inside the brain, like 

 that of the Ascidian Tadpole." 



This explanation may possibly be correct, but another explanation 

 appears to me possible, and I am inclined to think that the vertebi^ate 

 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 

 simultaneously 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 lace 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 ex- 

 ternal 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 transparency 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 



1 Degeneration, London, 1880, p. 49. 



272 



