282 JAMES ROLLIN SLONAKER 



lens cavity is represented by the line of junction of the anterior 

 and posterior walls (text-fig. 6, F and figure 48). This line of 

 junction is noticeable in all ages of the sparrow studied. Even 

 in the adult it is very conspicuous, especially in the equatorial 

 region. 



A noticeable difference in the shape of the lens of the sparrow 

 and that of the chick, occurs at this stage of development; 

 the sparrow lens is more spherical. The axial and equatorial 

 diameters in the sparrow form a ratio of 1:1.31, while in the 

 chick the ratio is approximately 1 : 2. The sparrow lens is more 

 nearly sjDherical, not only at this stage, but this condition obtains 

 throughout life. When the similar dimensions are compared in 

 the adults, the ratio in the sparrow is 1:1.56 and in the hen 

 1:1.73. These ratios vary with different species of birds. In 

 the seventeen different species which I have measured the ratio 

 of axial diameter to equatorial has ranged from 1:1.83 in the 

 bluebird (Sialia sialis) to 1:1.28 in the least sandpiper (Pisobia 

 minutilla). In the fox sparrow (Passerella iliaca) the ratio is 

 1:1.58. The average ratio of all these measurements is 1:1.58, 

 which is practically that of the EngUsh sparrow. 



From this time on to complete development of the lens the 

 most noticeable change is that of size. This is due almost 

 wholly to an increase in the lenticular portion by the addition 

 of new cells formed at the equator and applied to the surface of 

 the lenticular core in a parallel manner, thus forming successive 

 layers. These cells are long, six-sided, flattened structures 

 which extend from the anterior to the posterior surface. The 

 only part the anterior layer takes in increasing the size is at the 

 equator, where it is modified into the annular pad. 



The posterior surface shows the flattened appearance described 

 in the chick. This condition persists until about the eighth 

 day, when the anterior and posterior surfaces have about the 

 same curvature (text-fig. 6, 0). This relation of the curvature 

 of the two surfaces remains until about the age of hatching (text- 

 fig. 6, T), when the posterior surface has the greater curvature. 

 This condition persists throughout life. 



