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SYDNEY J. HICKSON. 
numerous that to attempt to refer to previous authors in the 
course of my description would only tend to hamper and 
obscure it. Consequently, I shall leave all reference to previous 
authorities to a separate section at the end of the paper, when 
I shall attempt to discuss the various disputed points and clear 
up the inconsistencies between my own descriptions and those 
of other observers. 
§ 1. The Eye and Optic Tract of Musca vomitoria. 
The eyes of the blow-fly are large brown protruding struc- 
tures situated upon the anterior surface of the head. Exter- 
nally they are protected by the chitinous cornese which are 
broken up into a large number of biconvex facets. The number 
of these facets varies in the individual. Muller (17) gives 
4000 for Musca domestica. In a vertical section through 
the middle of an eye of a blow-fly which had emerged from 
its pupa twenty-four hours, I was able to count 62 (PI. XY, 
fig. 2). In some I could count as many as 80, and in some 
only 40. The external convexity of the facet is usually formed 
of a greater arc of a sphere than the internal, so that between 
each facet there may be seen, internally only, a small flat surface. 
This is invariably covered with a dense pigment. This differ- 
ence between the external and internal convexity of the corneal 
facets gives a plano-convex appearance to sections that do not 
pass through their centres (v. PI. XV, fig. 3, c). 
Internally to the cornea is situated the so-called pseudocone, 
which is ensheathed by two or three nucleated pigment-cells 
(fig. 3 ,pg. x ). 
The pseudocone consists of four cells, each of which consists 
of a clear transparent external portion and a smaller proto- 
plasmic portion containing the nucleus situated internally 
(n.p. c). The clear transparent portion of each pseudocone-cell 
contains in the living eye a watery or perhaps slightly albu- 
minous fluid, for in specimens preserved in spirit all that can be 
seen of this portion of the cell is a band of protoplasmic 
substance stretching from the nucleus to the cornea, and 
staining deeply with hsematoxylon. Between each element of 
