THE EYE AND OPTIC TRACT OF INSECTS. 
235 
are in reality biconvex, the plano-convex appearance being seen 
only when the sections are cut not quite perpendicularly, or do 
not pass quite through the centre of the facets. Carriere (3) 
figures the facets of Musca plano-convex. Lowne (15), 
whilst figuring them biconvex as I have done, places them 
amongst what he calls the “ kistoid cornese.” The “ ‘ kistoid 
cornea * consists of a chitinous (?) articular membrane folded 
on itself so as to resemble a piece of honeycomb, the openings 
of the hexagonal cells being turned inwards.” “ The cavity 
of the corneal cell is occupied by the oil-like lens.” I have 
never yet been able to find anything that would answer to this 
description. I have never found anything like “ an oil-like 
lens,” nor can the cornea be truly compared with a honeycomb. 
Each facet in Musca is formed of chitin, the outer layers of 
which are the hardest and do not stain, whilst the inner ones 
are softer and frequently take up a considerable amount of 
haematoxylon or borax carmine staining. I have not made a 
special study of the development of the cornea, but since the 
publication of Mr. Lowne’s paper, I have re-examined many 
of my preparations of adult and immature eyes. As a result 
of this, I must express a very strong opinion that the facets are 
not developed from “ four original cells,” but from a con- 
tinuous sheath of protoplasmic substance which underlies the 
whole surface of the cornea, and which remains as a living 
protoplasmic lamina until the eye is fully developed and then 
shrivels up. That the so-called “ Semper’s nuclei ” have 
nothing whatever to do with the formation of the facets, 
I should think has been amply proved by the researches of 
Claparede (5), Max Schultz (IS), and Grenacher (7). 
My description of the ommatidia given above is almost 
identical with that of Grenacher’s ; the only point in which I 
am inclined to differ from him is in his description of the nuclei 
of the retinulae. 
He says : “ Von den Kernen der Retinula liegen sechs im 
vordern Drittel, wo sie (an geharteten Augen) buckelartige 
Auftreibungen verursachen, der siebente findet sich immer im 
hintern Drittel und gehort wohl zum Centralstabschen.” 
