Pc@^' 



Text fig. 6 Drawings of the developing lens from the early stages to the 

 adult, showing the rapid increase in thickness of the posterior wall of the lens 

 vesicle and relative lack of growth of the anterior wall. The distribution of the 

 nuclei at these various ages is represented by the dotted areas. The anterior 

 portion of each drawing is to the right. Drawings A to S, inclusive, refer to ages 

 of incubation; T to Y, after hatching. Ac, anterior wall; Ap, annular pad; ep, 

 ectoderm; L, lens vesicle; Lc, lenticular chamber; ope, optic cup; opv, optic 

 vesicle. 



A, chick 40 to 48 hours; B, chick 56 hours; C, chick 64 hours; D, chick 68 hours; 

 E, chick 90 to 100 hours; F, sparrow 2 days; G, sparrow 2§ days; H, sparrow 3 

 days; I, sparrow 4 days; J, sparrow 5 days; K, sparrow 6 days; L, sparrow 7 days; 

 M, sparrow 7J days; N, sparrow 7| days; O, sparrow 8 days; P, sparrow 9 days; 

 Q, sparrow 10 days; R, sparrow 11 days; S, sparrow 12 days; T, sparrow at hatch- 

 ing; U, sparrow nestling 2 days; V, sparrow nestling 4 days; W, sparrovv^ nestling 

 about 8 days; X, sparrow fledgling just flying; Y, adult sparrow. 



278 



