190 BULLETIN OF THE 
there are at least three kinds of cells. Two can be readily isolated ; the 
third has been studied only in sections. 
The retina extending from the line of deepest flattened nuclei to its 
outer margin breaks up into two very distinct forms of cells, — retinal or 
nerve-end cells, as Lankester and Bourne (’83, p. 182) have called them, 
and pigment cells. The retinal cells (PI. II. fig. 5) are elongated and 
rounded at their outer ends ; they terminate below in nerve fibres. From 
the rounded external end the calibre is uniform till the region of the 
thabdomeres is reached. Here the cells increase in diameter, and then 
continue for some distance uniform in size. Finally, each cell, enlarging 
slightly at its deep end, rapidly tapers into a nerve fibre. Throughout 
its whole extent the retinal cell contains pigment, which is principally 
concentrated, however, at its rounded outer end. 
The pigment cells (Pl. II. fig. 6) at their anterior ends, like the pig- 
mented tops of the retinal cells, abut against the preretinal membrane. 
From this they pass backward, and in the region of the rhabdome, where 
the retinal cells enlarge, they contract to thin fibres, which, after the 
rhabdome has been passed, again expand into irregular pigment sacs at 
the deep part of the retina. When isolated, they present the appearance 
(Pl. II. figs. 6, 7) of two sacs of pigment connected by a slender rigid 
fibre. 
The large round or slightly oval nuclei have been identified as be- 
longing to the retinal cells (Pl. II. fig. 7), and the smaller oval nuclei 
occupy the deep swollen ends of the pigment cells. It is possible 
that some of the pigment cells may not be prolonged in front of the 
rhabdomes, and therefore not possess anterior sacs; but I have never 
been able to discover such. The filamentous middle portion connecting 
the two extremities of the long pigment cells is so constant and char- 
acteristic in maceration preparations, that pigment cells which do not 
extend to the front of the retina must form the exception, if in fact they 
exist at all. 
Another method employed in studying the cells of the retina, and one 
especially instructive for the region of the anterior zone, was by the aid 
of sections perpendicular to the retinal cells. The retina has the form of 
a shallow bowl; consequently in sections perpendicular to its axis the 
deeper portions of the retina will lie at the periphery of the section, and 
its centre will be the region nearest the preretinal membrane. 
Figure 3 represents a portion of a retinal section whose centre, and con- 
sequently highest portion, is toward the right, and whose periphery or 
deeper portion is toward the left. The relatively higher portion of the 
