DEVELOPMENT OF EYE OF SPAEROW 297 



At the age of flying — about fourteen or fifteen days after 

 hatching — the crest is ahnost as densely pigmented as in the 

 adult. The other portions, however, lack the density aiid 

 gi'ouped arrangement of the adult. I am unable to state the 

 age at which complete pigmentation occurs, but it is some time 

 after the bird is flying and has been using its eyes. 



Pigmentation seems to be hastened and possibly started by 

 the light stimulus. It begins in the sparrow's pecten at about 

 the same time that the eye is exposed to the light, and in that 

 part of the pecten most exposed to the Hght. This is also true 

 of the chorioid in which pigmentation occurs first in the region 

 exposed to the light and later in the posterior portions. 



If light does act as a stimulus to the formation of pigment, 

 the relative delay in the development of it in the pecten of the 

 sparrow as compared to that of the chick may possibly be ac- 

 counted for b}^ the difference in the character of the nests of the 

 two birds. The sparrow's nest is a very compact and bulky 

 affair, often hidden in some recess or corner where the eggs and 

 nestlings are in semidarkness. The hen's nest, on the contrary, 

 is generally more exposed to the light and the hen's habit of 

 leaving the nest for a time during the warmest and brightest 

 part of each day would make the effect of light more pronounced. 

 A comparison with some other closely related bird with an open 

 nest would be of special interest in this regard. The fact that 

 in the chick the eye functions within a few hours after hatching, 

 while in the sparrow the eyes do not open until several days after 

 hatching and they are then not exposed to bright light until 

 about the time the bird leaves the nest, may also be a cause for 

 the delay of pigmentation in the sparrow eye and again points 

 to hght as an influencing factor. 



THE RETINA 



As previously described under the early formation of the eye, 

 the retina of the sparrow follows the same general order of 

 development as in other vertebrates. The retinal, or nervous 

 portion, is derived from the inner layer of the optic cup and the 

 pigment epithelium from the outer IsLjer. 



