BETINA OP THE BLOWFLY. 411 
sades, and ultimately pierce the basilar membrane and run 
between the great rods. 
The figure given (Plate XXVII. fig. 1) is from the eye of 
a Hawk-moth, in which these details are larger and more 
easily seen than in the Blowfly. The palisade bodies do not 
reach the basilar membrane, but are prolonged as extremely 
transparent rods, 3 to 5 /x in diameter, in the fly and in most of 
the insects I have examined, and from 20 to 30 [x in length 
(Plate XXVII. figs. 2 and 3, a). These with the palisade cells, b, 
form my bacilli or retinal end-organs, the whole length of which 
is from 60 to 70 y. The outer transparent portion is rarely 
straight, but usually strongly curved in a crook. They exhibit 
a fine longitudinal striation. 
The outer ends of the rods evidently consist of some substance 
resembling mucin ; they have the same refractive index and 
general characters as the mucin of the intestinal epithelial cells 
of the insect. 
The inner extremity of the outer part of the rod is imbedded 
in the fasciculus of elongate cell-like palisade bodies, fig. 2, 
which form the inner portion of the retinal end-organs ; each outer 
segment appears to be made up of a number of finer rods, 2 [x in 
diameter, pressed together into a cylinder; these produce the 
longitudinal striae. Each small component rod lies on the inner 
surface of one of the fusiform cell-like bodies which form to- 
gether the inner part of the retinal end-organ. 
The outer ends of the rods are surrounded and, except in 
very thin sections, concealed by the small round chaplet-cells of 
Viallanes (fig. 2, c ). These are connected with each other by fine 
processes and form a true adenoid sustentacular tissue, well seen 
in transverse sections of the pupa (fig. 4). 
Comparison of the Bacillary Layer with the Bacillary Layer 
of the Vertebrate Retina. 
In size and structure the elements of the retina are almost 
identical with those of the vertebrate ; the optic nerve terminates 
in the protoplasmic inner segment, whilst the outer segment is 
transparent, resists stains, exhibits longitudinal striee, and swells 
up with water in both. In both it is easily destroyed, and fre- 
quently exhibits vacuolation. 
LINN. JOUBN. — ZOOLOGY, YOL. XX. 
31 
