280 JAMES ROLLIN SLONAKER 



greater mass of these cells is located near the posterior axis of 

 the lens. These cells are arranged with their long axes parallel 

 to the surface of the lens. The thin stream of cells connecting 

 this mass with the outside mesenchjTae is closely applied to the 

 lens and is composed of a single layer of cells whose long axes 

 are also arranged parallel to the surface of the lens. This is 

 probably the beginning of the capsula vasculosa lentis, as well 

 as of the cells from which the vitreous body is developed. In 

 the sparrow eye I have seen no evidence of the capsule vasculosa 

 lentis. 



Very soon after the complete formation of the lens vesicle 

 the cells of the posterior wall begin to elongate and to extend 

 anteriorly into the lumen of the vesicle. This is seen in text- 

 figures 6, D and figure 46, which represent the condition of the 

 chick at the age of sixty-eight hours' incubation. This growth 

 is not uniform, but the cells in the axial region grow the fastest 

 and produce a convex inner surface. There is thus a gradual 

 transition from the long, fiber-like cells in the center of this 

 protuberance to the short cells at the equator of the lens. In 

 mammals the anterior wall of the vesicle is of uniform thickness 

 and becomes in the adult a very thin layer called the lens epithe- 

 Uum. In birds and reptiles the equatorial part of this layer 

 behaves in a different manner. It develops into the 'Ringwulst,' 

 or what I have designated the annular pad. This will be de- 

 scribed later. 



The posterior surface of the lens vesicle is now^ almost flat. 

 The thickness of the anterior wall is 0.036 mm. and the jDosterior 

 wall 0.096 mm. The axial and equatorial diameters have 

 relatively changed, giving a more lenticular shape to the lens. 

 The axial diameter is now 0.176 mm. and the equatorial 0.232 

 mm. — a ratio of 1:1.3. 



In the four-day chick (fig. 47) the cavity of the lens vesicle 

 is almost obliterated. The change of the shape of the lens is 

 very noticeable. The axial diameter is 0.160 mm. and the 

 equatorial 0.288 mm. — a ratio of 1 :1.8. The equatorial diameter 

 has increased much more rapidly than the axial diameter, 

 apparently because of the great increase in the number of cells 

 of the posterior wall. 



