310 
FREDA BADE. 
optic nerve. The pit is seen in sections to be a well-developed 
central fovea (136 jx deep at its centre), with a rather less 
well-marked though still distinct macula lutea surrounding 
it. The optic nerve has the usual position and relations, 
and both it and the yellow spot are clearly shown in fig. 1. 
An examination ot' vertical sections shows the usual eight 
layers of the vertebrate retina all well developed and with 
the normal characteristics. The retina is thickest around 
the central fovea, in the region of the macula lutea, and from 
there gradually decreases in thickness in all directions 
towards the ora serrata, where it changes its histological 
character, and the pars ciliaris retinae, consisting of columnar 
cells and a layer of pigment, begins (fig. 13). 
The thickness of the retina is as follows : 
(1) In the fovea centralis, 204 /x; (2) in the macula lutea, 
306 n; (3) half way between the fovea and the ora serrata, 
237 fx ; (4) in the pars ciliaris, 17 /x. 
As usual in the lower vertebrates no blood-vessels are 
present in any part of the retina. Quain (32) states that the 
Chelonia form an exception to this rule — an interesting fact 
in view of the supposed relationship between this group and 
the Rhynchocepliala. 
We may now consider the histological structure of the 
different layers of the retina separately. 
(1) Nerve- fibre Layer (numbered 1 in figs. 2, 4 and 12). 
Nerve fibres, of course, branch out from the optic nerve 
(fig. 1, o.n.) in all directions from its point of entrance, 
forming a very definite layer on the inner surface of the 
retina (figs. 4 and 12, I). This becomes thinner and thinner 
towards the front of the eye, disappearing altogether at the 
ora serrata (fig. 13, or.s.). The nerve-fibre layer is also 
absent from the central fovea (fig. 2, fx.) and is extremely 
thin over the macula lutea, where most of the other layers 
thicken around the central fovea (fig. 2, 1 ). One very 
definite band of nerve-fibres may be seen with the naked eye 
on the inner surface of the wall of the eyeball, commencing 
