AMPHIBIANS 



337 



The iris is thin and dehcate (Plate III). Both retinal layers are 

 i:)igmented and an ectodermal sphincter and dilatator of myo-epithelial 

 cells are present (Grynfeltt, 1906 ; Tretjakoff, 1900). The stroma is 

 thickly packed with melanophores as well as with cells containing yel- 

 low, brown and copper-coloured carotenoid pigments often associated 

 with a metallic sheen due to the presence of guanine crystals. As a 

 result the iris is usually In'illiantl}' coloured, as if dusted with a golden or 

 bronze powder so that it simulates the lustrous appearance of old gold 

 or Chmese lacquer ^ (Millot, 1923 : Mami, 1931). It is often coloured 

 similarly to the skin of ^^'hich at first glance it appears to be an integral 

 part. An almost constant feature is a vertical stripe often associated 

 with a groove running do\\7iwards from the lower pupillary margin to 



•^v*^V* p 



Fig. 399. — The Ocular Coats of Ra\a. 



Showing the rods and cones, r ; the pigmentary epitheUum, p ; the 

 choroid, c. divided into two strata ; the scleral cartilage, s ; and the sclera, sc 

 ( X 200) (Xorman Ashton). 



the periphery of the iris where it ends, presumably a relic of the ftetal 

 cleft (Johnson, 1927 ; Mann. 1931 ).2 The A^essels of the iris are 

 arranged in the same general ^Aan as those of Fishes : several superficial 

 arteries run irregularly and circumferentially on the surface taking a 

 tortuous course tow^ards the pupil and draining into vems which run 

 in a radial course but lie at a deeper level and are thus usually hidden 

 by the heavy pigmentation (Mann, 1929-31) (Plate III ; Fig. 402). In 

 the primitive clawed-toad, Xenojnis, all the vessels are obscured by 

 pigment. 



1 Some of the colours seen are brilliant and c^uite beautiful — gold and brown spots 

 on chocolate in the ecHble frog, Rana escidenta ; red and green dots in the American 

 bull-frog. R. catesbiana ; a chocolate stri^De across a gold and browni backgroimd in the 

 Malayan bull-frog, Kaloula puJchra ; gold speckled in russet in the tree-frog. Hyla, as 

 also in the giant toad. Bufo marinus ; gold and brown in the common toad, Bufo bufo ; 

 a brilliant metallic green in the gi-een toad, B. viridis ; green and silver with a horizontal 

 stripe of brown in the S. American toad, B. arenarum ; and so on. 



- Such a pigmeiated furrow or ridge, sometimes associated with a pupillary notch 

 is seen in certain teleostean Fishes (herring, trout, minnow, orfe. cod, carp. etc.). rarely 

 in Reptiles (the Bengal monitor, Varcmus bengalensis, and Igucnin titherrulald), and 

 never in Urodeles. 



S.O. — VOL. I. "- 



Xenopus 



