CYCLOSTOMES 



263 



vesicle ; the suggestion that the eye of the lamprey differed from all other 

 vertebrate eyes in that its elements were all autonomous in the sense that the 

 entire oi'gan arose from the neural ectoderm is, however, by no means proven 

 by the evidence submitted by this author and should be discarded. It is apposite 

 that in the still more primitive eye of the myxinoid, Bdellost07na, Stockard 

 (1907) found that the lens appeared in the usual vertebrate way as a vesicle 

 from the surface epithelium inrlependently of the optic vesicle. 



LIGHT-SENSITIVE CELLS. In the ojiidermis of the tail of the animoccotes 

 there are numerous iiiteresting cells cojiiously inncr\'ated from the lateral line. 

 Morphologically they resemble the ajiolar visual cells seen in tlie earthworm or 



¥ui 



s THiMncH THE l'<isTEi:iui; .Segment (.if 

 THE Fully Developed Ammoccetes. 



THE Eye of 



'/, ganglion cells ; /, internal nuclear layer ; c. external nucleai' layer ; v, 

 visual cells ; 7;, pigmented epithelium ; ch, rlioroid ; s, sclera ; m, muscular 

 tissue (Azan, X 250) (Katharine Tansloy). 



Mya (Fig. 86) and are said to be associated with a photosensitive pigment ; they 

 probably act as primitive ])liotoreceptors determining phototactic activity 

 (Young, 1935 ; Steven, 1950 51). It will be remeinbered that light-sensitive 

 cells of the type characteristic of Invertel)rates are also found ainong Chordates 

 in Amphio.ru.s as the cells of .loscphi ; anil it is interesting that the only other 

 Vertebrate which shows evidence (^f a similar primiti\e ])li()totaxis is the cave- 

 dwelling salamander, Proteus a>i</ii/)i us (Hawes. 194(i). 



THE LAMPREY EYE 



The eye of the lamprey and its relatives is of unusual interest in 

 that it shows a ninnber of primitive characteristics differentiating it 

 clearly from the eyes of Fishes and all other higher Vertebrates ; 

 nevertheless, it conforms closely to the essential structure of the eyes 



1 p. 229. 



