310 JAMES ROLLIN SLONAKER 



eye and the consequent total area of the retina do not reach the 

 adult condition until some time after the bird has left the nest 

 and is able to fly. Table 6 shows that the total area of the 

 retinal expanse at the age of flying is 77.410 sq. mm. This is 

 almost 8 sq. mm. less than that of the adult. The dimensions 

 of the eye in the axial and equatorial directions are correspond- 

 ingly less than that of the adult. 



The blood supply to the retina in birds differs from that in 

 mammals. The outer layers in each case are nourished by 

 osmosis from the vessels of the chorioid. The inner layers 

 receive their nourishment in a different manner. Early in the 

 development of the eye in mammals, the retinal artery makes 

 its appearance. This artery is distributed over the inner surface, 

 supplying all the layers of the retina, except the outer nuclear 

 and rod-and-cone layers. In the bird there is no retinal artery 

 nor do blood-vessels directly supply nourishment to the retina 

 as in mammals. The statement of 0. Hertwig that lampreys 

 do not have blood-vessels in their retina, but that all other 

 vertebrates do, is not true. In birds the inner layers of the 

 retina are nourished by osmosis from the blood-vessels of the 

 pecten. The development of the blood supply to the eye, 

 therefore, corresponds to the development of the pecten and 

 the chorioid. 



THE FOVEA 



The development of the fovea has been studied by a nmnber 

 of investigators. Chievitz ('87), who has made many observa- 

 tions on the embryonic development of the eyes of birds and 

 other vertebrates, states that the fovea begins as a thickening 

 of the retina. This thickened region he calls the area centralis. 

 In the center of this, area differentiation of the retinal cells into 

 layers is first observed. This region is later thinned out, or 

 pitted in by a radial migration of the cells from the center to 

 the sides. Numerous other investigators have verified these 

 facts. 



In my study of the development of the fovea of the sparrow 

 I agree in general with the results of Chievitz, but draw different 



