The Retina and Optic Ganglia in Decapods, especially in Astacus. 47 
averages from ten enumerations made on equal areas in tlie dorsal 
and in the ventral parts of the nerve, it was found that, when the 
nuclei in the ventral part were represented by 9, tliose in the dorsal 
part were represented by only 5.3. This difference leads me to 
believe that the sheath of Schwann are poorly developed in the 
dorsal part of the nerve and that, in consequence of this, the limits 
of the separate fibres are more poorly marked there than in the 
ventral part. 
Excepting the few fibres that enter the nerve directly from the 
second optic ganglion, all optic fibres take their origin, so far as I 
am aware, in the fibrillations of the fourth ganglion (PI. 2 Fig. 51) 
and terminate in a similar way in the optic lobes of the brain 
(Fig. 54). Whether there is a partial optic chiasma in the brain, as 
mentioned by several authors, I am unable to say. Although I 
have Seen appearances which indicate that there is one, in neither 
Golgi nor methylen blue preparations have I ever seen an optic 
fibre enter one side of the brain and pass across to the other side 
before breaking up into a fibrillation. Of the many preparations 
studied all have shown the optic fibres to terminate on the same 
side of the brain as that which they entered. 
The relation of the fibres in the optic nerve to ganglionic cells 
is not easily determined. In preparations of the fourth ganglion 
stained with methylen blue, the fibres of the optic nerve are seen to 
terminate in three ways: first, in a ganglionic cell of the large type 
and in a fibrillation (PI. 2 Fig. 51); secondly, in a ganglionic cell 
only (Fig. 52); and, thirdly, in a fibrillation only (Fig. 53). In the 
optic lobe of the brain these fibres, so far as my observations extend, 
always terminate in fibrillations (Fig. 54) and are unconnected with 
ganglionic cells. As every fibre very probably has a ganglionic cell 
directly connected with it, and as the region for these Connections 
seems to be the fourth ganglion, I have regarded preparations such 
as that represented in figures 53 as incompletely stained; and, had 
the process been completed, I believe a ganglionic cell would have 
been found connected with the fibre. Similarly, preparations such 
as those seen in figure 52 are probably instances in which the fibril- 
lation has failed to take up color, though the cell and fibre have. 
It seems to me probable that the whole truth of these relations is 
shown in figure 51 and that the other conditions (Fig. 52 and 53) are 
but partial representations of it. These observations lead me to 
conclude that those optic nerve fibres that connect the fourth gang- 
