130 



THE GENESIS OF THE VERTEBRATE EYE 



membranous labyrinth of the internal ear. It would be nice to be able 

 to insist that the lens placode has a real morphological existence and 

 that the lens is therefore a captured lateral-line organ, as Sharp be- 

 lieved; but we cannot do so with clear consciences. The best that can 



i/m 



Fig. 57 — Illustrating Tretjakoff's theory. From Walls, after TretjakofF. 



a, foveolcB opticas stage, b, stage of closed neural tube, showing hypothetical chorioid 

 plexus, cp. c, hypothetical stage in which the expansion of the chorioid plexus has created 

 the pigment epithelium and, by forcing the sensory retina to curve, is producing a two- 

 layered cup. d, stage in which the attachment of the cup to the skin is evoking a muscle, 

 m, and a lens, /; a remnant of the chorioid plexus forms the umbraculum, urn, corresponding 

 to the pupillary nodule of an amphibian, e, final condition of fish eye with free lens, /, 

 operated by retractor lentis muscle, Im; from the umbracular remnant um a lens may be 

 regenerated, as in salamanders (cf. Fig 106a, pn, p. 266). 



be said is that perhaps a former self-determination of the lens has been 

 replaced by a more convenient immediate chemical control by the optic 

 vesicle — just as the development of a secondary sexual character may 

 be under genetic control in one species of bird, while in another the 



