DEVELOPMENT OF EYE OF SPARROW 309 



dition. The inner segments are also thicker than in the adult 

 and are in direct contact with their nuclear portions. In no 

 case are the long, slender portions between the nuclei and the 

 cones, common to the adult, observed. The oil droplet common 

 to the adult cones is not visible. Distinctions between rods 

 and cones cannot be made in my sections. The fact that the 

 rods and cones are almost the last elements of the retina to 

 develop has been demonstrated by Hertwig and other investi- 

 gators in other animals. 



The migration of the pigment granules is becoming more 

 conspicuous. This migration appears as blunt, rounded pro- 

 jections which are so dense as to resemble a black mass. In 

 the six-day young these rounded pigment projections begin to 

 fray out at their inner ends so that some individual streams of 

 granules can be observed. 



About this time the eyes of the sparrow begin to open. The 

 most conspicuous change in the developing retina is the migra- 

 tion of the pigment granules. This is no doubt due to the 

 stronger light stimulus. The external segments of the rods and 

 cones are so masked by these granules that they are scarcely 

 visible. Very few rods are seen. The inner segments of the 

 cones are 0.004 mm. in diameter. Numerous twin cones are 

 seen at this age. In a tangential section the inner segments 

 of these twin cones measure 0.004 by 0.006 mm. in diameter. 



As development continues, the cones gradually recede from 

 'their nuclei, so that these structures are connected by the elon- 

 gated portion of the cell characteristic of the adult. 



The ganglion-cell layer shows the gradual thinning toward 

 the periphery until the adult condition is reached. That is, 

 it consists of a single row of cells which are relatively widely 

 separated from each other. 



The outer nuclear layer and the rod-and-cone elements become 

 reduced toward the periphery until the cones are relatively few 

 and not closely arranged. The cones are also shorter and thicker 

 at the periphery than at the optical axis. 



Though the retinal layers are differentiated into practically 

 the adult condition at a relatively early age, the full size of the 



