580 
MR. B. T. LOWNE ON THE MODIFICATIONS OF 
deeper structures of the eye, but it is perforated beneath each facellus, so that the 
latter is in continuity with the deeper structures. 
A membranous flask-like sac extends from the inner extremity of each facellus to 
the edge of the corresponding facet of the cornea ; this is lined with deep black 
pigment cells. 
Between each facellus and the ganglionic retina is a long compound rod, larger at 
its outer extremity than at its inner extremity. It is usually spoken of as the rod of 
the compound eye, but I shall call it the stemon (st), as I think I shall be able to 
show that it cannot be considered as the homologue of the rod-like structure of the 
true compound eye. 
The stemonata, corresponding to the outer facets of the eye (fig. 5), are very short 
and conical, being very much larger at their outer than at their inner extremities. In 
the immature imago the stemon can be seen to consist of seven cells, but in the 
mature insect, and especially in the centre of the eye, these are so perfectly fused 
together that the component cells of the stemon can be no longer recognised. 
The stemon is surrounded at its outer extremity by a very dense sheath of pigment, 
but this is deficient at its inner end. The stemon contains minute black granules of 
pigment, and these are arranged in four thread-like lines, which, with comparatively 
low powers, have an appearance which induced LeydigA to describe them as muscular 
elements ; beside these, minute scattered black pigment granules are seen in the pro- 
toplasm of the stemon. 
Some of the stemonata remain distinct throughout their whole course, but others 
unite with each other, so that three or four are fused at their inner extremities into a 
single thread. At their inner extremities all the stemonata branch, and are connected 
with pigmented stellate cells. 
The highly refractive threads of the facellus are seen in transverse sections at the 
outer end of the stemon, but I have been unable to distinguish them at the inner 
attenuated extremity. 
The stellate cells already alluded to are situated between two fire chitinous mem- 
branes. The outer of these (to 1 ) sends delicate sheaths over the stemonata ; the 
inner (to 3 ) is perforated by the fine processes of the stellate cells, which communi- 
cate with the round cells (g) within the second membrane. The round cells are 
supported in a fine network of neuroglia, also apparently given off from the inner 
surface of the second membrane. Beneath the round cells are several layers of 
fusiform cells (c), which appear to be situated at the outer extremity of the optic 
nerve, and to be in continuity with its fibres. 
III. On the Compoun d Eye of Yespa vulgaris and Vespa rufa. (Figs. 7 and 8.) 
I have used these two species indiscriminately in the investigation of the compound 
eye, as I have found no difference in its structure. 
* Loc. tit. 
