= 
VISUAL CELLS IN AMBLYSTOMA 507 
5. Rods do not appear in their definitive form until relatively 
late in development. ‘Their progenitors consist of large conical- 
shaped elements which possess, in early stages of development 
(fifteen days), an individuality by which they can be distinguished 
from the true cones. These elements are larger, their nuclei 
occupy a more external position than that of the cones, and the 
granular material of their outer segments becomes arranged in 
lamellae characteristic of the fully differentiated rod (figs. 5 and 
6). The ratio of these conical-shaped rod progenitors to that of 
the cones is approximately the same as that of the rod-cone ratio 
in the fully differentiated retina (viz., four to three). 
6. The two kinds of conical-shaped visual elements as seen 
in figure 4 (large and small) are regarded as visual cells of low 
specialization, which develop, respectively, into characteristic 
rods and cones by divergent differentiation (fig. 11). This inter- 
pretation is not in support of the view advanced by Cameron, 
who regards all of the conical-shaped visual cells in early stages of 
amphibian development as cones and the later appearing rods 
as transformed cones. 
7. Double cones appear very early in development (fig. 5). 
The absence of mitotic figures in the nuclei of the external nuclear 
layer during the period when double cones first appear suggests 
that they are formed from the fusion of single elements rather 
than from division. 
BIBLIOGRAPHY 
BERNARD, H. M. 1903 Studies in the retina. Q. J. Micros. Sci., N. S8., vol. 
43, pp. 23-48; vol. 44, pp. 443+468; vol. 46, pp. 25-75. 
Cameron, J. 1905 The development of the retina in Amphibia. Jour. Anat. 
and Physiol., vol. 39, pp. 135-153, 332-348, 471-488. 
1911 Further researches on the rods and cones of vertebrate retinae. 
Jour. Anat. and Physiol., vol. 46, pp. 45-53. 
Detwiter, 8. R. 1916 The effects of light on the retina of the tortoise and the 
lizard. Jour. Exp. Zodl., vol. 20, pp. 165-191. 
First 1904 Zur Kenntnis der Histogenese und des Wachstums der Retina. 
Lund’s Universitits Arsskrift, Bd. 40, adfeln 1, No. 1 (cited from 
Cayjale les oo8)- 
Kerr, J. GraAnAM 1919 Text-book of embryology, vol.2. London: Macmillan 
& Co. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 33, NO. 5 
