No. 1.] EVES OF ARTHROPODS. 139 
section of the optic ganglion. An outline drawing of the eyes 
in this position would then be almost the exact mirror-image or 
“Spiegelbild,” as the Germans call it, of the medulla of the 
optic ganglion (Fig. 1, wood-cut). 
Figure z.— A. Projection of the retinas just affer the rupture of the embryonic 
membranes. B. Horizontal section of the optic ganglion during the same period. 
I.-VII.=eyes. 6= cluster of inverted cells derived from sixth sense organ of eye 
VI.; d.a.= dorsal appendage; %. r. = horizontal retina; v. 7. = vertical retina; 
uv. f.= vertical fibre; 1 = plate of medullary substance between the vertical fibres, 
corresponding to a similar plate in the retina. mm. III. = medulla of eye III.; and 
m.d.a.= medulla of dorsal appendage, etc. The two cuts illustrate the corre- 
spondence between the structure of the medullz and that of the retinas. 
Medulla 3 (Fig. 48) is long and flat in section and somewhat 
pointed at the outermost end. It is divided into two unequal 
parts by a double row of coarse fibres. Between them is a clear 
space containing a long line of extremely fine medullary sub- 
stance, which I have not succeeded in dissolving into fibrilla, 
although I do not doubt it has such a structure from its great 
similarity to the medullary substance on either side of it. If we 
compare this part of the medulla with a horizontal section of the 
retinal furrow of eye III., we shall see (Fig. 59) that in both, 
there is a double row of coarse fibres seen in cross sections; 
the rows are separated by about the same distance, and between 
them is a thin layer of interwoven fibrillaz which in the retinal 
furrow are deeply pigmented. 
On either side of the double row of cut fibres, there is a layer 
