DEVELPOMENT OF EYE OF SPARROW 299 



the retina is relatively thinner at the anterior margin, it measures 

 more than at the three-day age. The pigment portion at the 

 lens measm-es 0.030 mm., the retina 0.056 mm., making a total 

 of 0.086 mm. At the axis of the eye these layers measure, 

 respectively, 0.009 mm. and 0.079 mm., making a total of 

 0.088 mm. 



The pigment cells have become elongated in the region of the 

 lens and have a few pigment granules in their inner ends. This 

 occurs only on the temporal side of the lens. These granules are 

 most abundant in the cells at the margin and grow less and less 

 toward the optical axis until they wholly disappear, slightly 

 posterior to the equator of the eye. This corresponds to the 

 condition of pigmentation found in the 7-to-9-mm. human embryo, 

 as described by Keibel and Mall. They say it develops first 

 in the pupillary border and progresses backward to the optic 

 stalk. They make no mention of pigmentation occurring in 

 one border before the other. The fact that the pigment appears 

 first in the region nearest the light indicates that, if not a cause 

 of its formation, it is at least a stimulus to hasten its development. 



The cells of the retina are beginning to migrate toward the 

 inner surface of the layer. This leaves a space in the outer 

 portion of the retina more or less free from nuclei. This space 

 varies in width; it is widest at the lens, where it measures 0.006 

 mm. From this region it diminishes gradually in width toward 

 the central part of the retina where it measures 0.004 mm. At 

 this age a few ganglion cells have partially differentiated in the 

 axial region. All of the other cells are elongated and spindle- 

 Uke in form and have the radial arrangement previously described. 



The fifth day of incubation shows some interesting changes 

 The combined thickness of the retinal and pigment portions 

 has increased. At the lens the total thickness is 0.084 mm. 

 and at the axial center it measures 0.116 mm. The dimensions 

 of the retinal and pigment portions of these two regions at differ- 

 ent ages of development are given in tables 4 and 5. 



All of the cells of the pigment layer now show pigment gran- 

 ules. In the cells near the lens the granules are situated at 

 the inner ends only. Slightly anterior to the equator of the eye 



