318 
FREDA BAGE. 
(1) The Ordinary Single Cones (figs. 3-9). 
These are most numerous. The outer segments (figs. 4, 5, 
o.s. 1 ) are conical in shape and in most of my preparations 
are more or less broken up into what appear to be flat, 
plate-like discs (fig. 5, d.o.s.). In quite a number of cases, 
however, part of the outer segment has the appearance of 
a closely wound spiral of two parallel threads (fig. 6, [b], 
s.o.s.). This is only the case with the inner half of the outer 
segment, i. e. the half next the oil-globule, and in these 
cases the outer half still appears as if broken up into discs 
(fig. 6 [b], d.o.s.). It is in preparations of the eye fixed in 
Flemming’s solution (fig. 6) that the spiral, black in colour 
owing to the osmic acid in the solution, is to be seen, and it 
seems to be due to shrinkage. Acetic bichromate material 
does not show it, only the discs being visible here in all 
parts of the outer segments (fig. 5, d.o.s.). They stain very 
slightly after this fixation, becoming pale yellowish in colour 
after picro-nigrosine or picro-indigo-carmine. 
A great deal of controversy has taken place over the 
structure of the outer segments of the visual cells of verte- 
brates. All workers are agreed in finding them very unstable, 
and no doubt the varying results obtained are due to the 
different appearances assumed after treatment of the various 
retinas with different fixing fluids and stains. 
Two main views are held as to the structure of the outer 
segments of cones. Many preparations have been obtained 
which show them broken up into cross discs (as in fig. 5, d.o.s.). 
This is given in the various text-books (Gamgee 29 , Halli- 
burton 31 , Quain 32 ). Heineraann ( 14 ) describes an annular 
breaking up into transverse plates in the outer segments 
of the cones in Chelonians, and Dogiel ( 11 ) states that the 
outer segments are ringed in some cases in Ganoids. The 
appearance seen in some of my preparations (fig. 6 [b], 
s.o.s .) , gives rise to another view, viz. that the outer segments 
are spiral. Hesse ( 15 ) gives an interesting historical resume 
