laXXEAX SOCIETV OF LOXDOX. 35 



Ui*. Gaskell unroi'tiiiiate!)' does not deal with the development 

 of the ])iiieal eye, wliich is far simpler. This has been carefully 

 studied is various types, all of which agree in essential features. 

 J myself have studied it chiefly in Sphenodon, upon which animal 

 the following statements are based. The pineal eye originates as 

 a simple evagiuatiou of the brain-roof. This completely separates 

 from the brain and closes up. The optic vesicle thus formed does 

 not invaginate to form an optic cup, as in the case of the paired 

 eye, but the retina, with its sense-cells, ganglion-cells and nerve- 

 fibres, is formed directly and in situ by differentiation of its 

 posterior wall, while the lens is formed from its anterior wall. 

 There is not the slightest indication of the origin of any part of 

 the retina directly from the superficial epiblast. It is true, of 

 course, that the whole of the central nervous system is derived, in 

 the first instance, from superficial e])ibiast, and so also is the 

 central nervous system of an Arthropod. Ko one denies that the 

 retina is epiblastic in orgiu ; the question is, what part of the epi- 

 blast is it derived from ? In the Vertebrate it is derived from 

 the part which becomes iuvaginaled to form the central nervous 

 system. In the Arthropod and in other Invertebrates, it is not. 



I cannot, therefore, avoid expressing the opinion that the 

 evidence which Dr. Gaskell derives from the study of the lateral 

 and pineal eyes in favour of his theory does not stand the test of 

 critical examination. It appears to me, if I may venture to say 

 so, that he has failed to distinguish between analogy and homology. 

 Animals which h;i,ve to live under similar conditions must be 

 expected to become adapted along similar lines, and it is no moi-e 

 necessarv to invoke a common ancestry to explain the resemblance 

 between the visual organs of Vertebrates and Arthropods than it 

 is to give the same explanation of the superficial resemblance 

 between their organs of locomotion. Again, the resemblance 

 between the lateral eyes of Vertebrates and the highly charac- 

 teristic compound eyes of any Arthropod is not nearly so striking 

 as is that betw'een the former and the higher Cephalopod eye, 

 and yet no one, so far as I am aware, has yet ventured to 

 include the Octopus in the ancestral portrait gallery of the 

 A'ertebraia. 



Looking at the problem for a moment from a wider point of 

 view, I should like to express my agreement with those who 

 see in Amphioxns a close approximation to the starting-point of 

 the great Vertebrate phylum. The evidence in favour of the 

 essentially primitive character of Antpldoxus is, to my mind, 

 overwhelming, but the acceptance of this evidence is fatal to 

 Dr. Gaskell's views, for in Amphioxint^ of course, a very large 

 proportion of the Vertebrate characters upon which he lays so much 

 stress as indicating Arthropod atlitiities, have not yet put in an 

 appearance. Thus, for example, there is no trace of either lateral 

 or pineal eyes, and we therefore conclude with confidence that 



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