500 S. R. DETWILER AND HENRY LAURENS 
unchanged, and when the retina is fully differentiated the rod 
nuclei are seen to constitute the outer row of the external nuclear 
layer. The same relationship between the position of the rod 
and cone nuclei was found in the eyes of the alligator (Laurens 
and Detwiler, ’21). In the human retina the cone nuclei occupy 
the more superficial position. 
In retinae of fifteen days the visual cells typically show two 
groups of granules (fig. 4), between which can be seen a clear re- 
Fig. 4 Developing visual cells from the retina of a larva of 15 days following 
the tail-bud stage of development. The larger elements are the progenitors of 
the rods; the smaller, of the cones. XX 1040. 
Fig. 5 Visual cells from the retina of a larva of 17 days following the tail-bud 
stage of development. The three large conical-shaped elements are differen- 
tiating rods. Their nuclei lie partly beyond the external limiting membrane. 
The granular material of their outer segments shows a lamellar arrangement 
characteristic of the fully differentiated rod (fig. 11). Arrow indicates double 
cone. X 1040. 
fractive disc marking the position of the oil drop. The original 
clear globule is retained in the proximal portion of the visual 
element and apparently develops the paraboloid. The proximal 
group of granules develops into the ellipsoid portion of the inner 
segment. The distal group of granules, as far as can be ascer- 
tained, becomes transformed into the typical granular discs 
which characterize the outer segment particularly in the rod. 
How this group of granules develops is uncertain from our prep- 
arations. That they may be separated off from the original 
