298 JAMES ROLLIN SLONAKER 



With two days' incubation these two layers show Httle or no 

 differentiation. They extend forward until they touch the lens. 

 Where they come in contact with the lens they seem to have 

 exerted sufficient force or resistance to growth to cause a bending 

 in of its surface. This has exerted an influence also on the retina 

 in that it tends to bend at the extreme margin. This is shown 

 in figures 31, 48, and 49. The combined thickness at the anterior 

 margin is 0.068 mm. and at the axial center 0.090 mm. 



The pigment layer is one cell deep and devoid of pigment. 

 It is thicker at the anterior margin (0.024 mm.) than at the axial 

 center of the eye (0.016 mm.). At the anterior margin it bends 

 inward and merges quickly into the retinal layer. 



The retinal layer, according to Low ('08), is many cells thick. 

 He bases his conclusion on the fact that the nuclei of this layer 

 are scattered uniformly throughout its entire thickness. The 

 cells are somewhat elongated and arranged with their long axes 

 in a fairly definite radial manner. On the other hand, Keibel 

 and Mall ('12), basing their conclusion on the position of the 

 mitotic figures, claim that these cells form a single layer and 

 that the nuclei alone are arranged in several layers. At the 

 anterior margin this layer measures 0.044 mm. and at the axial 

 center, 0.074 mm. 



In the three-day embryo little change is noted. The retina 

 is still in contact with the lens and shows the effect of pressure 

 as previously described (fig. 32); the pigment layer is without 

 pigment granules, and the nuclei of the retinal cells are still 

 distributed throughout the layer (figs. 50 and 51). A shght 

 difference is found in the thickness of these layers. The pig- 

 ment epithelium measures the same at the anterior margin as 

 in the two-day embryo, but is sHghtly thiner in the axial region 

 where it measures 0.012 mm. The retinal layer is somewhat 

 thicker, measuring 0.048 mm. at the lens and 0.076 mm. at the 

 axial center. 



Figure 52 shows the condition at four days' incubation. The 

 margin of the retina is distinctly thinned as compared to the 

 central region. It is still in contact with the lens, but does not 

 form a depression in its surface as in the earlier ages. Though 



