No. 3.] VISUAL CELLS IN VERTEBRATES. 57 
Between the outer and inner limbs is the intermediate plate 
(“Kittsubstanz,’ Schultze, 67, p. 218; ‘“‘Zwischenscheibe.” 
Greef, :00). This is unstained by the osmic acid and appears 
as a thin ight band in marked contrast to the black or brown 
of the outer limb. 
The inner limb shows a differentiation into a distal* 
ellipsoid and a proximal paraboloid (Krause). The ellipsoid, 
which has either the shape of the segment of a sphere, or is 
cylindrical, is distinctly granular. A sheath is not readily 
demonstrable in osmic-acid material, but is easily shown by 
other methods. 
The paraboloid is more transparent than the ellipsoid and 
is without evidence of granulation. It has a well defined 
inner clear portion and an outer less transparent sheath. Its 
form varies from ellipsoidal or paraboloidal to plano concave 
or plano convex, with the plane surface always distal. The 
flattened and concave conditions are probably distortions due 
to fixation. The paraboloid takes a selective purple stain, 
different from any other retinal structure, in preparations sub- 
jected to the following treatment. Material fixed 3 minutes in 
osmic acid vapor is prepared for treatment on the slide in the 
usual way: slides are placed in a % per cent. solution of per- 
manganate of potash for 20 minutes; then in a I to 5 per cent. 
solution of oxalic acid for 30 minutes. After washing, they are 
stained for 1 to 24 hours in Boehmer’s hematoxylin. The 
excess of stain is removed by 7o per cent. alcohol acidulated 
with hydrochloric acid and controlled by alcohol containing 
traces of ammonia. 
The nuclei (P1.'3, Fig. 20, st. nl. ex.), which in Necturus 
lie almost entirely proximal to the membrana _ limitans 
externa, are closely packed in a layer one deep in the func- 
tional part of the retina, but two deep toward the ora serrata. 
It is probable that extra-nuclear plasm from the inner segment 
is continued over the sides of the nucleus, but if so, it is 
* Proximal and distal are used throughout this paper with reference to 
the center of the eye. In the figures on the plates, proximal is above and 
distal below, except in detached portions of cells. 
