502 S. R. DETWILER AND HENRY LAURENS 
distinguishes them from the cone nuclei. The latter are oval in 
shape and occupy a deeper level. 
A count of the two kinds of visual cells shows that the number 
of larger elements slightly exceeds that of the smaller. The 
ratio is approximately the same as the rod-cone ratio in the 
fully differentiated retina, viz., four to three. 
Successive stages in the development of the retinal visual cells 
are shown in figures 7, 8, 9, 10, and 11. These figures show the 
developmental conditions at twenty-six, thirty-one, thirty-six, 
Fig. 6 Visual cells from the retina of an embryo of 21 days after the tail-bud 
stage of development. The nuclei of the differentiating rods are being drawn 
out on their inner pole, (cf. fig. 11). The cone nuclei are more deeply situated 
and are oval in shape. X 1040. 
forty, and fifty days, respectively. The difference in size be- 
tween the two types of cells gradually increases as development 
proceeds. The large conical element gradually becomes more 
and more rod-shaped, and the final typical rod form which it 
acquires is illustrated in figure 11. Throughout these successive 
stages, the nucleus of this visual cell (rod) not only becomes 
more and more drawn out on the inner pole, but it gradually 
increases in its stainability, as is shown in figures 10 and 11. 
These nuclei extend for variable distances beyond the external 
limiting membrane, whereas the cone nuclei lie beneath it. The 
outer segment of this element shows throughout a disc-like 
arrangement of its granular material. 
