DEVELOPMENT OF EYE OF SPARROW 



303 



Hertwig ('90). M. Schultze ('66) says that in rabbits and cats 

 (born with their Hds closed) the rods and cones are seen for the 

 first time soon after birth. In man they are formed two months 

 before birth (Keibel and Mall). 



TABLE 4 

 Shoiving the thickness at the axis of the different layers of the retina, the total thick- 

 ness, the thickness of the chorioid and sclerotic layers at different ages. Measure- 

 ments are in millimeters, and as nearly as possible perpendicular to the retina in 

 the axial region. 2 to 13 represent the ages in days of the embryos used; H-2, 

 hatched two days; H-4, hatched four days; Fov. c, H-fl, and Area H-fl, center of 

 fovea and area of young just flying; A-Fov, center adult fovea; A-a, thickness of 

 area of adult 0.360 mm. from center of fovea 



Differentiation into layers has proceeded much more rapidly 

 in the axial region than at the lens border. A gradual decrease 

 is noticed from the central part of the retina toward the peri- 

 phery. At the anterior margin it is impossible to distinguish 

 these layers (table 5). The undifferentiated cell mass here 

 fills the whole of the retinal layer. The margin of the retina 

 at the lens is relatively thin compared with the axial region. 

 It increases in thickness very slowly toward the macular region 



