494 S. R. DETWILER AND HENRY LAURENS 
From our observations several developmental possibilities were 
- suggested: first, if all of these typical cone-like elements are to be 
regarded as cones, then the rods, which do not appear in their 
distinctive form until later, represent transformation products of 
cones; secondly, that some of these elements functionally repre- 
sent rods even though possessing anatomical features character- 
istic of cones; and, thirdly, that all of these cone-shaped elements 
represent indifferent visual cells and the characteristic rods and 
cones come about by divergent differentiation of a visual cell of 
low specialization. 
In general, the rods are regarded as the more primitive type of 
visual cell (Graham Kerr, 719, and Parsons, 715) and the cones 
are regarded as specialized rods. If this be true, their appearance 
in ontogenetic development might be expected to antedate that 
of the cones—a condition just opposite to that which is suggested 
in the developing retina under consideration. The idea that 
cones represent specialized rods does not seem borne out by the 
condition found in many reptiles in which only cones are pres- 
ent (Detwiler, ’16). If they do represent specialized rods, then 
the early developing visual cells in pure cone retinae should have 
to be regarded as rods—a condition which seems very unlikely. 
According to Leboueg (’09), Magitot (710) and Seefelder (’10), 
the two kinds of visual cells in the human retina develop simul- 
taneously, and are mainly distinguishable by the fact that the 
axis of the diplosome (the centrosomes) is perpendicular to the 
long axis of the cone and parallel to the long axis of the rod. This 
is not entirely borne out in Seefelder’s illustration which is re- 
produced in figure 12. The cones would also seem to be dis- 
tinguishable by their much larger size. Cajal (’11) says that cones 
and rods evolve in essentially the same manner and it is difficult 
to distinguish them at the beginning (see also Fiirst, ’04). 
Bernard (’03) and Cameron (’05 and ’11), who studied the de- 
veloping retina in amphibia, conclude that cones represent early 
stages in the formation of rods. Their conclusions are based on 
the fact that in early stages of the developing retina only conical 
elements are to be seen, while in later stages typically shaped 
rods predominate, 
