No. 3.] VISUAL CELES IN. VERTEBRATES. 585 
designated by German authors as Haupt-Zapfen and Neben- 
Zapfen respectively. The elements are united to each other 
along one side from the region of the membrana limitans 
externa to their ellipsoids. 
I have been unable to distinguish any difference between 
the outer segments of the double cones and those of single 
ones. 
As the ellipsoids of the pair do not stain alike, they differ 
chemically and this suggests a functional difference. That 
they are morphologically equivalent, or homologous, no one 
would question. In the far cone the ellipsoid takes the same 
color as it does in the single cone. That of the near cone 
shows much less affinity for stains, in this respect resembling 
thereliipsoid: of the rod) (Pl.1, Fig. 2;/ Plas; Pigs 338). 
The paraboloids display the most conspicuous difference 
seen between the cell organs of the pair. That of the far 
cone is slightly smaller than the ellipsoid while the paraboloid 
of the near cone is three to four times as large as its own 
ellipsoid and looks like a large flask-shaped vesicle. 
The myoid*of the far cone. (Pl. 1, Fig. 2, mys com. dst) 
is very long, and reaches from the nucleus to the ellipsoid. 
In the myoid dark staining fibrils can be seen more plainly 
than in any other element. At the distal end, the fibrils pass 
around the paraboloid. Their farther course seems to be 
the same as in the single cones, where they have their origin 
on the outer limb. Seen in optical section the fibrils seem 
to end on the paraboloid in small bead-like enlargements, 
but by focussing, these are apparently resolved into a con- 
tinuation of the fibrils (Pl. 1, Fig. 2, cp.). This appearance 
may be due to the fact that these fibrils are more distinct 
in cross section than when seen !engthwise. At the proximal 
end of the myoid the fibrils pass over the appropriate nucleus, 
as described for the other elements, and into the outer reticular 
layer. 
The fibrils of the near cone lie embedded in the sheath of 
the large paraboloid, but otherwise their course is like the 
