VISUAL CELLS IN AMBLYSTOMA 499 
comes about through partial vacuolization of the cytoplasm. 
Occasionally, small refractive granules can be seen in the clear 
globule. These may represent ingested pigment, but the heavily 
stained ones situated distal to the globule and which comprise a 
large part of the visual cell, certainly give no indication of origi- 
nating from the pigment epithelium. 
Leplat (713) has investigated the réle of mitochondria in con- 
nection with the development of the rods and cones in the chick. 
He also found deeply stained granules in the initial protoplasmic 
buds. These granules, which he has called mitochondria, are to be 
seen in figure 13A, which is a reproduction of one of his illustra- 
tions. They are seen to bear a striking resemblance to those 
which we have observed (figs. 2 and 3), yet our preparations were 
fixed with the aid of 5 per cent acetic acid, which is a mito- 
chondrial solvent. Leplat regards the transverse striation of the 
external segment and its cleavage into dises (fig, 18 B), as the 
expression of its mode of construction from mitochondria. In 
our preparations a similar transverse arrangement of the material 
is readily seen (fig. 5), though they cannot be regarded as mito- 
chondrial granules for reasons previously stated. They may, 
however, represent chemical transformation products of mito- 
chondria with the retention of the plan of the mitochondrial 
arrangement. 
No evidence of rod and cone identity based on the posture of the 
diplosome could be gathered from our preparations. In iron- 
haematoxylin stained sections, the protoplasmic buds exhibited 
only large groups of granules as seen in figures 2 and 3. If a 
typical diplosome is present, such as is shown in Seefelder’s 
illustration of the developing human retina (fig. 12) and Leboueq 
(709) and First (’04), it is entirely masked by other granules. 
Leboucq and Seefelder describe the cone bud as being much 
stouter than that of the rod (fig. 12). This figure also shows that 
the nuclei of the cones lie in a plane closer to the external limiting 
membrane than the rod nuclei. In Amblystoma the initial pro- 
toplasmic buds are approximately of the same size. ‘The visual 
elements in a retina of several days’ further development show 
marked differences in size (fig. 4). This relationship remains 
