VISUAL CELLS IN AMBLYSTOMA 495 
The difficulty in accepting the conclusions of Cameron and 
Bernard lies in the fact that the cone-like elements in the early 
stages, which they call cones, may functionally represent rods 
even though possessing the anatomical configuration characteris- 
tic of cones. The variability in the shape of the visual cells has 
made absolute rod and cone distinction difficult in some cases. 
We have already referred to this difficulty in an earlier paper 
(Laurens and Detwiler, ’21, p. 225). 
From our first survey of the developing retina in Amblystoma 
the conditions appeared to bear out the conclusions of Cameron 
and Bernard. However, more critical examination has shown 
that, although all the elements are cone-like in early stages of 
development, they possess characteristic features by which they 
ean be divided into two groups, one of which can be readily iden- 
tified as the progenitors of the definitive rods, the other of the 
cones. It seems hardly wise to regard the forerunners of the 
rods as functional cones even though possessing certain cone- 
like features. 
METHODS 
Our observations were made upon the retina of Amblystoma 
punctatum. The larvae were preserved in corrosive sublimate- 
acetic fixing fluid at successive twenty-four-hour intervals for a 
period of twenty-six days following the tail-bud stage of develop- 
ment. From this period on, specimens were fixed at successive 
intervals of from three to five days. Sections were cut 8» in 
thickness and stained in Ehrlich’s haematoxylin and erythrosin, 
iron haematoxylin and eosin, and in Held’s molybdic acid haema- 
toxylin. 
OBSERVATIONS 
The visual elements make their initial appearance in retinae 
of approximately eleven days after the tail-bud stage of develop- 
ment. They are first seen in the posterior portion of the optic 
cup, just above the point of entrance of the optic nerve—a region 
corresponding to the optical axis of the eye (fig. 1). Prior to this 
141] references to ages are based upon days after the tail-bud stage of develop- 
ment. ; 
