DEVELOPMENT OF EYE OF SPARROW 283 



Accompanying this change in relative curvature of the two 

 surfaces, there is also a change in the ratio of the length of the 

 axis to the equatorial diameter. Table 2 shows that there is an 

 increase in the ratio from an almost 1-to-l relation in the two- 

 day stage to a 1-to-l. 56 ratio in the adult. In other words, there 

 has been a gradual change from the slightly flattened spherical 

 shape to the lenticular form. It also shows that the increase in 

 equatorial dimension is due to the development of the annular 

 pad rather than to much change in the ratio of the axial and 

 equatorial diameters of the lenticular portion. 



In the two-day embryo the nuclei of the cells of the anterior 

 layer are scattered throughout the layer, while those of the 

 posterior layer are situated about midway of the length of the 

 cells. The position of the nuclei is represented by the dotted 

 areas in the drawings of text-figure 6. 



As the lens increases in size, the thickness of the anterior layer 

 remains practically the same. In other words, the increase in 

 size of the lens is largely due to the growth in length and increase 

 in number of the cells of the posterior layer, but the nuclei of 

 the lenticular portion become more and more inconspicuous 

 until they have almost disappeared by the twelfth or thirteenth 

 day (text-fig. 6, T), the age of hatching. Two daj^s after hatch- 

 ing (U) the lenticular part is almost entirely free from nuclei, 

 except a few cells which join the annular pad. They have either 

 disappeared or have changed so as no longer to take the stain. 

 The nuclei of the anterior layer are conspicuous until about the 

 time the bird leaves the nest. Clear and unobstructed sight 

 can therefore not be possible in the sparrow until about the age 

 of flying. 



A better idea of the relative growth of the different parts of 

 the lens can be obtained from the following table of measure- 

 ments. All measurements were made with the eyepiece micro- 

 meter and of sections as nearly as possible through the center 

 of the lens, along the axis of vision. 



This table shows almost a uniform increase in the dimensions 

 of the different parts. Some variations occur, especially in the 

 dimensions of the annular pad and the anterior layer. These 



