finds that infection taken in by the nymph can be transmitted by the adult, a fact
which somewhat confused my earlier interpretations of the cycle of development.
Later work has made it quite clear that club shaped bodies may arise as a result
development of the forms seen in the blood either in the gut of the adult or in that
of the nymph. In the adult they find their way to the ovaries and oviducts, where
they may sometimes be seen in swarms, and finally embed themselves in the ova.
In the nymph they wander indiscriminately into the tissues and embed themselves
in the protoplasm of the embryonic tissue cells which are already beginning to shape
the future adult. Whether in the ovum or in tissue cell the club shaped body
swells up and becomes a characteristic round body which for convenience of
description and because it offers certain analogies with this stage of the malarial
parasite I have called the " zygote ".
The zygotes quickly undergo a great increase in size, reaching eventually as
much as 25μ . I n t h e i r g r o w t h t h e y e x h i b i t s o m e c h a n g e s i n t h e c h r o m a t i n o f a
very peculiar nature. Eventually they split up into a number of bodies which I
have for the time termed " sporoblasts " and these again sub-divide to form bodies
resembling very closely piroplasma in the blood, which since they are found collected
in the salivary glands I have called "sporozoits".
Thus far, supposing fertilisation to take place prior to the formation of the
club shaped bodies, for which there is a certain amount of evidence, piroplasma in
the main outlines of its development resembles the malarial parasite, but in certain
details and in the relation of the parasite to the tissues of the host there are some
striking differences.
Only in a few cases do zygotes as in the malarial parasite go through division
into sporozoits in the cell originally invaded. Much more commonly the sporo-
blasts become separated by the division of the rapidly growing embrionic cells, and
possibly also by their own movements, so that the sporozoits when formed are
more or less disseminated through the tissue.
In the developing adult, parasites are commonest in the embryonic tissue which
will eventually form the salivary gland, and when sporozoits are formed they are ready in sitn
in the cells of the salivary acini. But is quite common for parasites to invade
other embryonic cells than those which will eventually become salivary cells and
possibly in this case the sporozoits reach the gland by their own movements. The
final result is the packing of many of the salivary cells with thousands of sporozoit
forms. In a previous publication I called attention to the fact that the nymphal and
adult glands contained forms which appeared to be of different types, This difference
is, however, not a very fundamental one and is due only to the fact that in the salivary
glands of the nymphs I had then examined the process of sub-division into sporo-
zoits was not complete. In my previous note I described from a wrong interpre-
tation of the nature of the " sporoblasts " what seemed to be two cycles leading to
the formation of sporozoits, but it is clear that one cycle only exists as described.
Stated briefly the hereditary cycle of piroplasma is as follows:—
A parasite in the gut of the tick enlarges and becomes a motile club-shaped
body, which then leaves the gut and penetrates an ovum becoming in the substance of
this a separate zygote. The zygote increases in size and breaks up into sporoblasts
which are found disseminated in the tissues of the larva. In the glands of the
nymphs immature sporozoits have collected. In the glands of the adult are found
sporozoits.
The nymph to adult infection is identical. Club shaped bodies are formed
in the gut. They leave this and penetrate the tissues, embedding themselves in
cells and becomin
g
zygotes. Owing to the growth of the embryonic tissue the
sporoblasts into which the zygote breaks up tend to become disseminated among
the cells. Many of these cells later form the salivary gland and the sporozoits
then find themselves in situ. Others probably reach the glands by their own
movements.