VISUAL CELLS IN AMBLYSTOMA 501 
group by liquefaction is suggested by observations which show 
that in the center of the single group of granules a clear area 
(granule-free) can be seen as shown in figure 3. Whether or not 
this is actually the case, cannot be definitely determined. At 
any rate, the clear refractive oil drop or dise which separates these 
two groups of granules gives no indication of coming from the 
original globule, which persists in the proximal portion of the 
inner segment as the paraboloid. 
Double cones can be seen in early stages of development, as 
illustrated in figure 5. These double elements give no indications 
as having arisen by division of single elements. When they are 
first observed, each member possesses its own nucleus. The 
total absence of mitotic figures at this stage when double cones 
first make their appearance suggests that they result from the 
fusion of preexisting single elements rather than by division. 
In the retina of seventeen days (fig. 5), the inner and outer 
segments of the visual cells are readily distinguishable. The 
inner group of granules has enlarged and is more deeply stained. 
They constitute the ellipsoid portion of the inner segment. The 
granular material of the outer segment of the large conical 
visual cells has undergone cleavage into discs which are separated 
by clear non-stainable bands. The lamellar arrangement of this 
granular material is quite prominent in the outer segment of the 
large visual cells, but less so in the smaller ones (fig. 5). The 
difference in the structural arrangement of the material in the 
outer segment which can be observed at this period serves as 
another index that the individuality of the larger conical-shaped 
elements is different from that of the smaller ones. Although 
these large elements are conical in shape, the structural arrange- 
ment of the material in the outer segment is quite similar to that 
which characterizes the fully differentiated rod, and all the 
evidence suggests that these large elements are not to be regarded 
as cones. 
In embryos of twenty-one days (fig. 6) the visual cells show 
increased growth. The nuclei of the large conical-shaped ele- 
ments (differentiating rods) begin to assume a pear-shape—a 
condition which, added to their more external situation, readily 
