108 PATHIEN. [ Vor.aik 
two twisted cells (Pl. X., Fig. 58, a. d.). The inner end of one 
of the component cells is swollen, and contains a round, fairly 
well-stained nucleus; the outer end is quite small, and its con- 
tracted tip terminates in a flattened rod. The second cell is 
perhaps a little smaller; its broad outer end contains a very 
faintly stained nucleus, and also terminates in a flattened rod. 
Its opposite extremity fuses with the first cell to form the in- 
ward prolongation of the retinophora. Both cells supplement 
each other’s irregularities so perfectly that a symmetrical and 
apparently single cell is the result (Figs. 57a and 58a). 
When the cells are pigmented, the position of the second 
nucleus is often plainly indicated by a clear spot, but it would 
not be possible to identify it as a nucleus until the pigment had 
been removed. 
The outer ends of the two component cells of the retinophorze 
are widely divergent (Fig. 58, ¢). A similar condition occasion- 
ally obtains in Avca and Haliotis, and this fact affords excellent 
proof of the double nature of the retinophore. 
PIGMENT. — Just after the rupture of the embryonic mem- 
branes, one can readily distinguish with the naked eye the 
bright red ocelli of the living embryos. The pigment of these 
stages is readily soluble in alcohol, the reddish-brown pigment 
found in the succeeding stages, much less so. It is the latter 
pigment that is first seen in sections, distributed in coarse gran- 
ules through the iris. At about the same time, the outer ends 
of the retinal cells assume a diffuse reddish-brown color. This 
coloring matter was probably in distinct granules, and became 
diffuse through the action of the alcohol. 
In the next stages, thin sections show a row of minute, pig- 
mented blocks, arranged in pairs (Fig. 69). The blocks are 
cross sections of a pigmented collar surrounding the outer end 
of each cell. 
In the fully developed eye, the pigment varies in color from 
brown to jet black, according to the method of preparation. 
Chromic acid, bichromate of potash, and Muller’s fluid, partly 
dissolve the pigment, leaving it a light brown color. Picro-sul- 
phuric acid has a similar, but much less effect. Eyes treated 
with alcohol alone contain great quantities of intensely black 
pigment, which in all stages is most abundant around the outer 
ends of the retinal cells, 
