THE COMPOUND EYE OF THE BLOW-FLY. 529 
The pseudo-cone lies immediately beneath the corneal lens, 
and the outer end of the axis of the rhabdome is imbedded in 
its apex. 
The Great Rods (Pl. XXXVI., Fig. 1, rh) occupy the greater 
part of the dioptron; each rod extends from the apex of a 
pseudo-cone (c, c) to the basilar membrane (m b), and consists 
of an axial rhabdome (rh) and its sheath. Five pigment cells, 
the iris cells (2), surround its outer extremity ; a similar number 
(n*) are seen at its inner extremity on the basilar membrane. 
Both sets of cells give off pigmented fringes, which lie upon the 
surface of the great rods, those of the iris cells meeting and 
interlocking with those of the inner pigment cells. 
The great rods in transverse section (Pl. XXXVI, Fig. 2) 
exhibit a distinct lumen, surrounded by a transparent wall. 
The wall stains readily with carmine stains, and has the 
appearance of granular protoplasm ; it is not stained by nuclear 
stains. 
On the inner surface of the transparent wall there is a deli- 
cate, apparently cuticular membrane, resembling the intima 
of a tracheal tube; but, instead of transverse thickenings, it 
exhibits six or seven longitudinal threads. In the recent state, 
and in well-preserved sections, the lumen of the great rod is 
very distinct, but there is a strong tendency for the sheath to 
contract; in many transverse sections this contraction is so 
marked that the axis of the great rod appears to be completely 
occupied by the highly refractive dots, which are the transverse 
sections of the fine longitudinal threads. Under these circum- 
stances the great rods are seen to be separated from each other 
by considerable spaces, but in good preparations they are only 
separated by very narrow ones, owing to their larger diameter 
and the considerable lumen they enclose. 
Grenacher’s figures almost invariably represent the lumen 
more or less contracted by the infolding of the inner cuticular 
layer. 
Very generally in the fully formed imago each great rod 
exhibits a single nucleus, midway between its ends, but in an 
earlier stage there are usually several nuclei at its extremities. 
35—24 
