EVOLUTION AND SIGNIFICANCE OF FOVEA 111 



Both WooUard and G. Elliott-Smith have emphasized 

 the existence of the so-called area centralis retinae in the 

 eye of Tarsius as a forerunner to the development of the 

 anthropoid macula and fovea. With respect to this region 

 Woollard says: "This differentiated area Hes on the lateral 

 side of the entrance of the optic nerve, that is in the position 

 of a macula, and represents the macula of Tarsius. It is 

 possible to arrange in a series the transformations taking 

 place in this region of the retina for the enhancement of 

 vision. The first degree shows a gradual increase in all 

 the layers of the retina. The second a well-defined area in 

 which this increase takes place. Finally, an area in which 

 a fovea is formed and only cones are present, the other 

 layers being pushed aside in order not to interfere with the 

 transmission of light. Tarsius overlaps the first and second 

 in having a well-defined area in which the layers have 

 increased. The cones, however, are absent and a fovea is 

 not formed." The difficulty of accepting Woollard's findings 

 and interpretations is based upon two main facts. First, 

 this differentiated area centrahs, according to him, consists 

 of a series of localized thickened convolutions in the retina. 

 If these localized convolutions or folds which he describes 

 in Tarsius represent a real area centralis, it is difficult to 

 understand how such a region could function for increased 

 visual acuity. A marked localized increase in the number 

 of retinal elements in the visual axis of the eye does provide 

 for enhanced visual acuity, such as in the turtle, but it 

 would seem that the existence of retinal convolutions or 

 undulations in such a region could do no other than bring 

 about a marked distortion of the image in the same way 

 that a wavy photographic plate would destroy definition. 

 Secondly, since Tarsius is strictly a nocturnal animal possess- 

 ing rods only, it seems questionable that a real area centralis 

 should be present on these grounds alone. In so far as I am 

 aware, areas and foveas do not exist in strictly nocturnal forms. 



In my own studies upon the nocturnal lemurs (Galago and 

 Nycticebus) I have observed retinal convolutions or folds, 



