246 THE EYE IN EVOLUTION 



vestigial eye. He attempted to overcome the second difficulty by 

 postulating a migration of the lens from the cerebral to the superficial 

 aspect of the vesicle ; his conception of the evolution of the organ is 

 seen in Figs. 259 to 262. If the vertebrate eye stems from an ascidian- 

 like ancestor in this way, the formation of the tubular neural structure 

 precedes sensory differentiation, and any superficial sensory organ asso- 

 ciated with the surface ectoderm must be assumed to disappear and be re- 

 placed by the establishment of a neural photosensory organ. Why the dor- 

 sal and lateral areas of the neural tube should show this photosensitive 

 differentiation raises a difficult problem ; as occurs in many Inverte- 

 brates, the tendency may be associated with orientation to light coming 

 from above, the paired lateral areas being evolved primarily in relation 

 to orientation in the horizontal plane. The analogy, however, is by no 

 means proved or even clear, and the danger of phylogenetic deductions 

 in such a case is obvious ^ ; in Froriep's (1906) view a common ancestry 

 is more probable than a sequential derivation. 



A further hypothesis, the placode theory, usvially credited to von Kvipffer 

 (1894), was suggested by Nuel (1887) and supported by Beraneck (1890), 

 Burckhardt (1902) and Lubosch (1909). It postulated the development of 

 ectodermal placodes homologous with the lateral line organs from the anterior 

 members of which the olfactory organs, the membranous labyrinth of the ear and 

 the Jens of the lateral eyes were developed. The lens was originally vesicular 

 and was considered to form an eye with a verted retina ; the definitive retina 

 emerged from the central nervous system to act as its optic ganglion, homologous 

 with a spinal ganglion, and eventually as phylogenetic evolution proceeded, 

 took over the sensory function of the lens which degenerated into a dioptric 

 accessory. This theory, however, has long been in disrepute since no evidence, 

 ontogenetic or phylogenetic, connects a non-sensory retina with a sensory lens. 



The origin of the lens^ — the other major factor in the development 

 of the vertebrate eye — has also given rise to speculation. The 

 homologous position of the olfactory and otic anlages suggested first to 

 Sharp (1885) that this structure arose from an ectodermal placode and 

 was in its own right a sensory structure. Without attributing photo- 

 sensitive properties to its cells as called for in the preceding theory of 

 retinal development, several investigators have been attracted to the 

 view that the lens is an independent organ derived from an anterior 

 placode of the epibranchial series (Jelgersma, 1906 ; Studnicka, 1918 ; 

 Schimkewitsch, 1921). The evidence of experimental embryology is 

 conflicting. Many experimenters have established that the presence 

 of the optic vesicle is necessary for the development of the lens, and 

 some liave claimed that this structure alone is sufficient for its deter- 

 mination so that a lens will form from undifferentiated ectoderm at an 

 abnormal site if the optic cup is transplanted thereto. Others have 



1 Seo -vritings of Balfour (1878-81), Metcalf (1906), Keibel (1906), Buxton 



(1912), Bti ; 921), and others. 



