34 PltOCKEDlNCS OK Tin: 



support a llifory it looks as il' th.-n theory must slaiul upon a 

 soniinvliat shaky fouiuhition. 



Dr. GaskpU, then, coiiclutk's that iu the pineal t-ve of Ammocmtes 

 "there is certainly no api)earance in the least resembling a compound 

 retina such as is seen in the vertebrate or crustacean lateral eye.'' 

 It is true that in the Lampreys tlie retinal ganglion of the pineal 

 eye is not spread out to form a layer of such unilonn thickness 

 as in the lateral eye, but the pineal eyes of Sphenodon and of the 

 Lacertilia make a much closer approach to the lateral eyes in this 

 respect. 



By far the most important evidence afforded by both the pineal 

 and lateral eyes of A'ertebrates, however, is, in mv opinion, that 

 derived from their development. Both differ essentiallv from 

 any Invertebrate eye in being formed as diverticula of a hollow 

 brain. The eyes of Arthropods are formed by thickening and 

 differentiation of the superlicial epiblast. How is it possible to 

 reconcile this discrepancy ? Dr. Gaskell himself {op. cit. p. 101) 

 states the problem quite clearly in the case of the lateral eyes. 

 Having arrived at the conclusion tliat the retina is in this case a 

 compound retina, composed of a retina and retinal ganglion of the 

 type found in Arthropods, he gops on to say : "From this it follows 

 that the development of the vertebrate retina ought to show the 

 formation of (1) an optic plate formed from the peripheral epi- 

 dermis and not from the' brain ; (2) a part of the brain closely 

 attached to this optic plate forming the retinal ganghon, which 

 remains at the surface when the rest of the optic ganglion with- 

 draws : (3) an optic nerve formed in consequence of "this with- 

 drawal, as the connection between the retinal and cerebral parts 

 of the optic ganglion." Of course, the same must apply to the 

 pineal eyes *. 



Itelyiiig upon Gotte's observation " that the retina arises from 

 an optic plate, being the optical portion of his ' Sinnesplafte,' " 

 Gaskell concludes that the retina (of the lateral eve) is to 'be 

 regarded as a portion of the superficial epiblast together with 

 a retinal ganglion with which it has become fused, while the 

 optic vesicles are explained as outgrowths of the primitive 

 Arthropod stomach which supply only the epithelial and supporting 

 framework of the retina, with which the nervous and sensory 

 elements become interwoven. The development of the lateral 

 Vertebrate eye is, however, a very complex process, and as I have 

 not made a special study of it myself, I leave it on one side, 

 though I may say that Dr. Gaskell's idea of the double origin of 

 the retina and its supporting structures seems to me to be too far- 

 fetched to be of much value as a support for his theory, and that 

 any attempt to institute a close comparison between the lateral 

 eye of a Vertebrate and the highly specialized compound eye of an 

 Arthropod is foredoomed to failure. 



* At any rate so far as no. (1) is concprned, whatever view we may take as 

 to the presence or absence of a retinal ganglion in the pineal ej e. 



