10
been evacuated either naturally or artificially, we shall, as a rule, find no trace at
all of the parasites. Sometimes, however, we do find an occasional small bolus
of pigment or some scattered granules of it ; but these will be seen on careful
examination to lie invariably in the cavity, in association with such small traces
of the former meal of blood as have not succeeded in escaping from it—as
indeed was to be expected. But the tissues themselves, whether the cells, the
homogeneous outer coat, or the muscular coat, disclose no sign whatever of the
parasites or of their typical pigment ; they remain as entirely free from such as
if the insect had been fed on healthy blood, or not fed at all ; and it is quite
clear that in insects fed as just described, the remains of the parasites which
have escaped digestion have been voided amongst and as part of the normal
excreta.
If, however, we now examine at the same stage a grey mosquito fed on
blood containing mature proteosoma, a very different spectacle awaits us. On
looking: at the tissues of the stomach we shall find the external muscular and
homogeneous coats to be studded with small clusters of pigment identical in ap-
pearance with the characteristic pigment of proteosoma,
These constitute the youngest forms of the pigmented cells yet observed
by me. It is necessary to examine them very particularly as regards (a) their
position in the tissues, (b) their appearance and structure.
With a little skill, it is very easy to distinguish the various structures of
Which the mosquito's stomach is composed, especially when measures have been
taken to prevent the cover-glass from unduly flattening out and crushing the
tissues. On depressing the focus of the oil-immersion lens we shall come upon
successively, first the ramifications of the air-tubes, with clusters of oil-globules;
then upon the crossing bands of muscular fibre ; then upon the outer homo-
geneous coat, in which no appearance of cellular structure is observed ; then
upon the layers of distinct cells ; and lastly upon the cavity, containing bacteria
or traces of blood. Going further down we shall reach the other side of the
stomach, with the cells, the homogeneous coat, the muscular fibres and the air
vessels, all in a reverse order.
We shall find, then, no difficulty in locating the young pigmented cells-
They lie invariably either in the external homogeneous coat or, still further
outwards, actually between the strands of muscular fibre, which they separate in
a manner with which we are familiar-in the case of trichina embryos (Plate I,
figure 5). I have never yet found them lying in or amongst the cells of the
stomach ; still less in the innermost layer of cells which we may perhaps consi-
der as epithelial cells, or in the cavity of the stomach itstlf. This rule appears
to be without exception. If the pigmented cells do not actually lie between the
muscular fibres, they lie just beneath them (Plate III).
In appearance, the most elementary forms seem at first sight to consist of
nothing but clusters of about 20 grains of pigment arranged within a small oval
area, the longer diameter of which is about 8 μ, and the shorter diameter
6 μ or 7 μ. The pigment granules are however often seen to be arranged on
the circumference of the oval ; from which fact taken together with the observa-
tion that all similar clusters in the same insect appear to be of much the same
size, we infer the fact that the pigment is contained within a cell. This is easily
confirmed by further observations in which the containing cell is clearly visible.
In fact, taking insects killed on the morning of the second day, we shall
generally find several varieties of these youngest pigmented cells. One variety
presents the appearance just described of a mere cluster of pigment (Plate I,