RETINA OF LATERAL EYES OF SPHENODON PUNCTATES. 315 
(7) Layer of Cones (numbered 7 in figs. 2 and 4). 
There are no rods present in Sphenodon, in which respect 
this animal agrees with lizards, snakes, and tortoises. Indeed 
the absence of rods is common to all reptiles, as stated by 
Gatngee (29). The cones are of several distinct kinds, both 
single and double being present in large numbers, and in 
some cases much smaller ones (figs. 8, S.S.C.) occurring 
between the ordinary single ones (fig. 8, O.S.C.). This layer 
is, of course, made np only of those parts of the visual cells 
which project beyond the membrana limitans externa, the 
portions which contain the nuclei lying in the outer nuclear 
layer, as already stated. It is continuous over the Avhole of 
the retina proper (except, of course, the blind spot). The 
cones measure in length from 37 to 50 jj. , the different sizes 
being intermingled with one another over the greater part 
of the retina as shown in fig. 5. Towards the ora serrata 
they become shorter and thicker, and gradually diminish in 
number until they completely disappear in the pars ciliaris. 
Over the macula lutea they are somewhat longer and corres- 
pondingly thinner than in any other part, and more closely 
packed together. As might be expected the most densely 
packed cones are the longest and narrowest, so that at the 
central fovea there are fifteen cones to every six in the main 
part of the retina, the corresponding number in the macula 
lutea around the central fovea being eight. 
Over the macula lutea the pigment epithelial layer of the 
retina, usually so easily separated, remains attached to the 
cones, as clearly shown in figs. 1 and 2. 
(8) Layer of Pigment Epithelium (numbered 8 in 
figs. 2 and 4). 
Each cell of this layer consists of an outer portion which 
abuts against the choroid, from which it is separated by a 
very distinct membrane, and an inner portion broken up into 
long processes (fig. 4, p.p.) which pass inwards between the 
outer segments of the cones. The outer portions fit closely 
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