THE VERTEBRATE EYE 



245 



Tretjakoff (1913), Hescheler and Boveri (1923) and Nowikoff (1932). It 

 was assumed that these cells became orientated in a regular manner 

 with their associated pigment cells towards the central canal, and then 

 were carried towards the skin in company with paired lateral diverticuli 

 of the neural tube (Figs. 255 to 258). It is to be noted that by this 

 hypothesis the inversion of the retina and the position of the pigmentary 

 epithelium are also well explained. Although objections have been 

 raised to this conception, such as the lack of ontogenetic and phylo- 

 gent^l- confirmation of any intermediate stages of the migration, the 



Figs. 259 to 262. 



-Froriep's Derivation of the Ascidian (and Verte- 

 brate) Eye. 



Fig. 259. 



Fig. 260. 



Fig. 261. 



Fig. 259. — The hypothetical original exi.stence of two sensory vesicles 

 with an external lens and verted retina. 



Fig. 260. — Involution of the neural tube showing a lens facing the neural 

 canal and a verted retina. 



Figs. 261 and 262. — Degeneration of one eye of the original pair and 

 migration of the lens to an external position. For siinijlicitj- the statocyst 

 portion of the sensory vesicle is omitted. 



absence of Hesse's cells in the head-end of Amphioxus and the danger 

 of phylogenetic deduction from a species which appears to be an off- 

 shoot rather than a primitive t\^e, the theory is undoubtedly ingenious. 

 On the other hand, a phylogenetic analogy with the vesicular eye 

 of the ascidian tadpole ^ was suggested by Lankester (1880) and 

 strongly advocated by Jelgersma (1906). Such an ascidian hypo- 

 thesis had to meet the criticism that this eye is unpaired while the 

 presence of a lens situated on the cerebral aspect of the retina is 

 obviously an anomaly (Fig. 235). Froriep (1906), however, suggested 

 that the first difficulty could be overcome if the apparently unpaired 

 eye in reality represented one of a pair ; in support of his hy|3othesis 

 he showed that it was situated asymmetrically towards the right and 

 was balanced by a degenerate mass on the left which he interpreted as a 



1 p. 228. 



