ESTERLY: EUCALANUS. 23 
(88-94, p. 361), also, uses practically the same expression, though he 
considers it as unsettled whether the nerves leave the cells from the 
inner or the outer ends. And Hesse (:01 and :02) states many times 
that the median eyes of Crustacea are inverted. 
As far as my observations on Eucalanus elongatus go, I have been 
able to confirm in every respect the statements of Grenacher (’79) 
concerning the relation of the optic nerves to the retinal cells. 
As already stated, the bundle of fibres (axis cylinders) composing 
the optic nerve leaves the eye directly at its posterior border, dorsal 
to the basal plate of the ventral eye and between the posterior basal 
plates of the lateral eyes. ‘This may be readily determined from whole 
preparations (Fig. 1, n. opt.), and is seen in sagittal sections (Plate 1, 
Fig. 6, n. opt; Plate 5, Figs. 44, 46, 48) and frontal sections, though 
more clearly in the former. If the section coincides precisely with the 
sagittal plane, the basal plates of the lateral eyes are not cut, but the 
relation described above will appear in cross (Plate 5, Fig. 49) or 
frontal sections (Plate 1, Figs. 2, 5). Immediately behind the eye 
(Plate 4, Figs. 38, 39) the optic nerve is circular in cross section, 
but within the eye it is separated into parts, which have come from 
the two lateral portions and the single and ventral portion of the eye 
(Plate 1, Figs. 2, 5, 6). ) 
g. Numerical Relation of Nerve Fibres and Visual Cells.— The 
number of fibres in the optic nerve corresponds precisely with the num- 
ber of retinal cells in the eye as a whole. ‘This can be so readily seen 
in cross sections of the nerve (Plate 4, Figs. 38-43) and has been ob- 
served in so many cases that there can be no doubt on that point. In 
the region of the optic nerve behind the eye before the frontal nerves 
have joined it (Plate 1, Fig. 1; Plate 4, Fig. 38) the fibres are closely 
massed into a single cylindrical bundle, but posterior to this region the 
fibres gradually become more widely separated from one another 
(Plate 4, Fig. 39; Plate 5, Fig. 47). About half way between the eye 
and the brain and thence to the brain, the cross section of the nerve is 
very much flattened dorso-ventrally and elongated laterally (Plate 4, 
Figs. 40, 41). In any cross section of the nerve posterior to the point 
at which the frontal nerves (n. f.) join it, the latter are always distin- 
guishable by the presence of a fibre which has a delicate sheath stain- 
ing black in vom Rath’s fluid (Plate 4). 
Near the eye, the fibres of the optic nerve are rounded in cross 
section, and each is provided with a delicate sheath, which, though 
distinguishable from the axial bundle, is very closely applied to it 
(Plate 4, Figs. 38, 39). But farther from the eye, the sheath and the 
