THE VERTEBRATE EYE 



247 



found that a lens may partially or completely develop if the retinal 

 anlage has been removed from the optic plate at an early stage or in 

 anencephalic monsters.^ It may well be that there is some tendency 

 for the formation of a lens inherent in the ectoderm of the region where 

 it is normally found ; but on the whole, in the present stage of our 

 knowledge, the evidence would seem to suggest that this structure is 

 secondarily formed, called into existence normally by two mutually 

 reinforcing inductors — the cells of the optic vesicle and the mesoderm 

 of the head — although in certain experimental conditions no further 

 stimulus beyond that provided by the latter may be necessary (Twitty, 

 1930-55 ; Woerdeman, 1950 ; Liedke, 1951). 



It would seem, therefore, that despite the considerable amount of 

 thought expended on the question, the emergence of the vertebrate eye 

 with its inverted retina of neural origm and its elaborate dioptric 

 mechanism derived from the surface ectoderm, is a problem as yet 

 unsolved. Indeed, appearing as it does fully formed in the most 

 primitive species extant today, and in the absence of transition forms 

 with which it can be associated unless by speculative hypotheses with 

 little factual foundation, there seems little likelihood of finding a 

 satisfying and pragmatic solution to the puzzle presented by its 

 evolutionary development. 



Balfour. J. Anat. Physiol., 9, 128, 408 



(1878). 

 A Treatise on Comparative Embryology, 



London (1881). 

 Beckwith. J. e.rp. ZooL, 49, 217 (1927). 

 Benoit. Bull. Biol. France Belgique, 71, 



393 (1937). 

 Beraneck. Arch. Sci. phys. nat., Geneve, 



24, 361 (1890). 

 Boveri. Zool. Jb., Suppl. 7, 409 (1904). 

 Biitschli. Vorlesiingen it. vergl. Anat., 



Berlin (1921). 

 Burckhardt. Verh. int. Zoologencotig. 



(1901), 621 (1902). 

 Buxton. Arch, vergl. Ophthal., 2, 405 



(1912). 

 Choi. Folia anat. japon., 10, 29 (1932). 

 Dohrn. Mitt. zool. Stat. J^eajoel, 6, 432 



(1885). 

 Estable. Ayx. Inst. Neurol., Montevideo, 



1, 328 (1927). 

 Franz. Bolk's Hb. d. vergl. Anat. d. 



Wirbeltiere, Berlin, 2 (ii), 989 (1934). 

 von Frisch. Pfli'igers Arch. ges. Physiol., 



138, 319 (1911). 

 Froriep. Hertwig'a Handbnch d. vergl. und 



exper. Entwicklungslehre d. Wirbel- 

 tiere, Jena, 2 (1906). 

 Anat. Anz. 29, 145 (1906). 

 Hagedoorn. Arch. Ophthal. (Chicago), 16, 



783 (1936). 



Hescheler and Boveri. Vjschr. naturf. 



Ges. Zurich, 68, 398 (1923). 

 Jelgersma. Morphol. Jb., 35, 377 (1906). 

 Keibel. Klin. Mbl. Augenheilk., 44 (2), 



112 (1906). 

 von Kennel. Diss., Dorpat (1881). 

 Krause, W. Arch. mikr. Anat., 11, 216 



(1875) ; 12, 742 (1876). 

 von Kupffer. Studien zur vergl. Entwick- 



lungsgeschichte d. Kopfes d. Kranioten : 



II. Die Entwicklung d. Kopfes v. 



Ammocoetes planeri, Munich (1894). 

 Lange. Zbl. prakt. Augenheilk., 32, 131 



(1908). 

 Lankester. Darwinisyn and Partheno- 

 genesis, London (1880). 

 Quart. .J. micr. Sci., 31, 445 (1890). 

 Leboucq. Arch. Anat. micr., 10, 555 



(1909). 

 Leplat. C. B. Ass. Anat., 17, 194 (1922). 

 Lewis. At)ier. J. Anat., 3, 505 (1904). 

 Liedke. J. exp. Zool., 117, 573 (1951). 

 Lubosch. Morphol. Jb., 39, 146 (1909). 

 Mangold. Ergebn. Biol., 7, 193 (1931). 

 Metcalf. .4/ia^ .4h;., 29, 526 (1906). 

 Miiller, W. Beit, zur Anat. unci Physiol. 



{Festgabe C. Eudwig), Leipzig, 2 



(1875). 

 Nowikoff. .4cac/. Tcheque d. Sci., Bull. 



internat. Clin. d. Sci. math., nat. med., 



33, 131 (1932). 



» See Lewis (1904), Keibel, (1906), Srockard (1910), Spemann (1912), Leplat (1923), 

 Beckwith (1927), Mangold (1931), Choi 1932), Waddington and Cohen (1936). 



