THE COMPOUND EYE OF THE BLOW-FLY. 535 
Blow-fly and in those insects in which the chaplet cells are 
most conspicuous ; or they may be true nervous elements. 
My own opinion inclines entirely to the first of these views. 
I regard them as mesoblastic elements which form the outer 
surface of the neuroblast from which the retina is developed, 
and which grow in between the bacillary elements, and form a 
kind of sustentacular framework. They are possibly analogous 
to the pigment epithelium of Vertebrates, although, like the 
choroid of the latter, they have a mesoblastic origin. 
In support of this view, I may mention that they first appear 
as an invagination (Fig. 63, p), and that their behaviour with 
stains is somewhat different to that of the nerve cells of the 
ganglia. They colour more feebly. In specimens prepared with 
gold chloride they exhibit the form of a continuous network 
(Pl. XXXVIII., Fig. 5, /), and in their deeper layers this is easily 
seen in tangental sections; they are entirely replaced by fringed 
pigment cells in noctuid Moths and in many other insects. 
It must be admitted, however, that, tempting as this explana- 
tion of their nature is, the observations of Viallanes and of 
Hickson lead to a different conclusion. Viallanes states that 
they form chaplets and are not connected with each other 
transversely, and Hickson gives some remarkable figures from 
specimens fixed with gold chloride and subsequently teased out. 
I confess I have been unable to obtain specimens such as he 
figures. His figures are, however, very remarkable [237, Figs. 
16 and 17). What he terms a neurospongium is evidently, I 
think, a tracheal capillary network, but he represents fine fibres, 
which he regards as nerve fibres, connecting these cells on the 
one hand with the optic nerve, and on the other with the so- 
called palisade cells of Carriére, my proximal segments of the 
bacilli. 
The Pigmented Tapetum——One of the principal objections 
advanced against the views here advocated is that in most 
insects a thick layer of opaque pigment intervenes between 
the palisade layer of Carriére and the membrana basilaris. 
This is especially the case in noctuid Moths. This pigmented 
layer takes the place of the chaplet cells, and, according to my 
