REPTILES 



(Franz, 1913 ; JokL 1923). Considerable variations occur in size and 

 shape. As a rule it is a relatively simj)le structure and only in some 

 Iguanids (particular!}- Conolopkus and Metopoceros, Plate VI) does 

 it become plicated and approach the complexity and beauty of the 

 pecten of Birds. It may be circular in cross-section, oval, X- or Y- 

 shaped (as in the monitor lizard. Varanus) ; it may be short and 

 stumpy, forming a small cushion-like paj^illa on the disc, as in nocturnal 



363 



"^^'i?»i.^>**-ii^' 



Fig. 430. — The Retina of the Lizaed. 



(1) ganglion cells ; (2) inner plexiform layer ; (3) inner nuclear layer ; 

 (4) outer plexiform layer ; ('^) outer nuclear layer ; (6) visual cells ; (7) pig- 

 mentary epithelium ; (8) choroid ( X 500) (Xorman Ashton). 



forms (most geckos; the leaf- footed lizard, Pygopus) or in the chameleon, 

 or long and slender pointing toAvards the centre of the globe (the slow- 

 worm, A7iguis fragilis), sometimes nearly reaching the lens (the green 

 lizard of Southern Europe, Lacerta viridis) ; only in the degenerate 

 burrowing types (Ampliisbaenidae, etc.) is the conus lacking.^ 



In its histological structure the retina itself is avascular, thick and 

 richly cellular with a well-defined lamination (Fig. 430) ; the inner 

 nuclear layer with 9 or 10 rows of superimposed nuclei is compact and 

 the ganglion cell layer with. 2 or 3 rows of cells is particularly well- 

 developed and conspicuous. The visual cells in most species are of two 



1 p. 733. 



Anguis 



