DEVELOPMENT OF EYE OF SPAEROW 281 



The cells of the posterior wall continue to grow in length, to 

 become more fiber-like, and to push forward the inner surface 

 of the wall until the cavity of the vesicle is completely oblite- 

 rated by the union of the posterior and anterior walls. This 

 occurs in the chick, according to Froriep ('06), in the second half 

 of the fifth day of incubation. At this age he gives the following 

 dimensions: axial diameter, 0.29 mm.; equatorial diameter, 0.57 

 mm.; thickness of anterior (epithehal) wall, 0.018 mm.; posterior 

 wall, 0.272 mm. The ratio of the two diameters is now about 

 as 1:2. The lens has thus changed greatly in shape from the 

 almost spherical form found in the early stage of the lens vesicle 

 to the lenticular shape seen when the cavity of the lens vesicle 

 has been completely obliterated. In other words, the axial 

 and equatorial diameters have changed from approximately a 

 1:1 to a 1:2 ratio, respectively. 



According to Rabl ('00), in man, after the lens fibers have 

 reached a length of about 0.18 mm., they no longer divide, but 

 simply grow in length. An increase in the number of lens fibers 

 occurs at the junction of the anterior epithelial layer with the 

 posterior portion near the equator. 



If the same be true in the development of the lens of the pig, 

 this length of lens cells is found in the 11-mm. embryo. This 

 corresponds very closely to the stage of the sixty-eight-hour 

 chick when these cells are 0.076 mm. long. My sections do not 

 show whether mitotic division is still occurring in these cells. 

 It appears, however, that cell division is taking place more 

 rapidly at the periphery than at the center of this protuberance. 

 After complete obliteration of the lens Vesicle, there is no doubt 

 that in the sparrow new lens fibers are formed near the equator 

 of the lens at the junction of the lenticular part with the epi- 

 thelial layer. 



The development of the eye has now reached a stage which 

 corresponds very well with the earliest age secured of the spar- 

 row. This was at about two or two and one-half days' incu- 

 bation. If the rate of development is proportionally the same 

 in the two birds, the first stage secured in the sparrow is between 

 two and three days. At this age all that is left of the original 



