RETINA OF 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 /i in diameter, in the fly and in most of 

 the insects I have examined, and from 20 to 30 /u in length 

 (Plate XXYII. figs. 2 and 3, a). These with the palisade cells, 5, 

 form my bacilli or retinal eud-organs, the whole length of which 

 is from 60 to 70 ji. 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 o£ 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 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 Itetina. 



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 striae, and swells 

 up with water in both. In both it is easily destroyed, and fre- 

 quently exhibits vacuolation. 



LINN. JOURN. ZOOLOar, VOL. XX. 31 



