268 



THE EYE IN EVOLUTION 



sutures (Capraro, 1 934-37 ).i It has a light yellow coloration derived 

 from a pigment the composition of which is unknown (Plate, 1924 ; 

 Franz, 1932 ; Walls and Judd, 1933). 



The retina, even at this early stage of Vertebrate evolution, shows 

 the essential architecture of the vertebrate eye ; but is entirely avas- 

 cular and without any suggestion of an area centralis (Fig. 280). 

 Next to the outer layer of pigmentary epithelium lie the visual elements, 



^^=' 



M»»^-.^ 



Fig. 280. — The Retina of Lampetra fluviatilis. 



g, ganglion cells ; i, internal nuclear layer, consisting essentially of 

 horizontal cells above and bipolar cells below ; e, external nuclear layer ; 

 ?», visual cells ; p, pigmented epithelium (Feulgen, X 370) (Katharine Tansley). 



thereafter their nuclei form an outer nuclear layer which is followed 

 by a combined layer containing bipolar cells, horizontal cells, amacrine 

 cells and a few sparse ganglion cells. 



The nature of the visual elernejifs has given rise to some controversy, 

 but most authorities are now agreed that even in this, the most primitive 

 of Vertebrates, two types of cell exist, a relatively long and a relatively 

 short cell, the former with a voluminous ellipsoid and short external 

 segment, the latter with a smaller ellipsoid capped by a longer external 

 segment. The differentiation between the two types and their relative 

 numbers vary in different families (Walls, 1035). In the primitive 



1 Compare the lens of lizards, p. 361. 



