THE RETINA 17 



bleaching out the pigment. The same may be said of the 

 teleosts. In reptiles, the amount of pigment varies consid- 

 erably. Turtles possess a moderate amount, diurnal and 

 nocturnal lizards are more heavily pigmented, and especially 

 is this true of the fovea of diurnal forms. In the alligators and 

 crocodiles, the melanin pigment varies in amount and dis- 

 tribution. Below and immediately above the optic nerve 

 disc it is heavy. In the dorsal and posterior part of the eye, 

 melanin pigment is almost absent, but instead, the cells here 

 are filled with fine guanine granules which give this portion 

 of the retina a glistening silvery appearance in contrast with 

 the dark brown of the lower portion of the eye. This guanine- 

 containing portion constitutes a retinal tapetum in contrast 

 with the pigment-free choroidal tapetum of mammals 

 (Figure 14). A retinal guanine tapetum occurs also in 

 numerous fishes (Wunder, 1926). 



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ABC 



Fig. 14. Drawings of various regions of the epithelial pigment layer of Alliga- 

 tor mississippiensis. A, a portion of the tapetum showing broad zone of guanine, 

 and a narrow vitreal border of melanin; B, a portion of the pigment layer 

 showing choroidal guanine-containing portion and vitreal melanin-containing 

 portion; C, epithelial pigment layer beyond the region of the tapetum, showing 

 dense melanin pigment and absence of guanine pigment. X 665. (Redrawn 

 from Laurens and Detwiler, 1921, /. Exp,. Zool., v. 32.) 



The significance of the epithelial pigment, its amount 

 and migratory capacity in relation to vision or to adaptation 

 of the eye, is a subject about which much has been written. 

 This subject will be considered in detail in a later chapter. 



Pars Optica Retinae. The retina in the more limited sense 

 consists of a very delicate membrane covering the inner 



