The nuclear changes and multiplication which precede
sporoblast-formation vary greatly in different Gregarines and can only
be outlined here. In the formation of both sets of sexual elements
(gametes) there is always a comprehensive nuclear purification or
maturation. This elimination of a part of the nuclear material (to be
distinguished as trophic or somatic, from the functional or germinal
portion, which forms the sexual nuclei) may occur at widely-different
periods. In some cases (_Lankesteria_, _Monocystis_), a large part of
the original (sporont-) nucleus of each associate is at once got rid
of, and the resulting (segmentation-) nucleus, which is
highly-specialized, represents the sexual part. In other cases, again,
the entire sporont-nucleus proceeds to division, and the distinction
between somatic and germinal portions does not become manifest until
after nuclear multiplication has continued for some little time, when
certain of the daughter-nuclei become altered in character, and
ultimately degenerate, the remainder giving rise to the
sporoblast-nuclei (_Diplodina_, _Stylorhynchus_). Even after the
actual sporoblasts (sex-cells) themselves are constituted, their
nuclei may yet undergo a final maturation (e.g. _Clepsydrina ovata_);
and in _Monocystis_, indeed, Brasil (2) finds that what is apparently
a similar process is delayed until after conjugation and formation of
the zygote (definitive sporoblast).
Nuclear multiplication is usually indirect, the mitosis being, as a
rule, more elaborate in the earlier than in the later divisions. The
attraction-spheres are generally large and conspicuous, sometimes
consisting of a well-developed centrosphere, with or without
centrosomic granules, at other times of very large centrosomes with a
few astral rays. In those cases where the karyosome is retained, and
the sporont-nucleus divides up as a whole, however, the earliest
nuclear divisions are direct; the daughter-nuclei being formed either
by a process of simple constriction (e.g. _Diplodina_), or by a kind
of multiple fission or fragmentation (_Gregarina_ and _Selenidium
spp._). Nevertheless, the later divisions, at any rate in _Diplodina_,
are indirect.
By the time nuclear multiplication is well advanced or completed, the
bodies of the two parent-Gregarines (associates) have usually become
very irregular in shape, and produced into numerous lobes and
processes. While in some forms (e.g. _Monocystis_, _Urospora_,
_Stylorhynchus_) the two individuals remain fairly separate and
independent of each other, in others (_Lankesteria_) they become
intertwined and interlocked, often to a remarkable extent
(_Diplodina_). The sexual nuclei next pass to the surface of the
processes and segments, where they take up a position of uniform
distribution. Around each, a small area of cytoplasm becomes
segregated, the whole often projecting as a little bud or hillock from
the general surface. These uninuclear protuberances are at length cut
off as the sporoblasts or gametes. Frequently a large amount of the
general protoplasm of each parent-individual is left over unused,
constituting two cystal residua, which may subsequently fuse; in
_Diplodina_, however, practically the whole cytoplasm is used up in
the formation of the gametes.
[Illustration: After Leger, from Lankester's _Treatise on Zoology_.
FIG. 13.--Development of the Gametes and Conjugation in _Stylorhynchus
longicollis_.
a, Undifferentiated gamete, attached to body of parent-individual.
b-d, Stages in development of motile male gamete.
e, Mature female gamete.
f, g, Stages in conjugation and nuclear union of the two elements.
h, Zygote (copula).
i, Spore, still with single nucleus and undivided sporoplasm.]
The sporoblasts themselves show all gradations from a condition of
marked differentiation into male and female (anisogamy), to one of
complete equality (isogamy). Anisogamy is most highly developed in
_Pterocephalus_. Here, the male elements (microgametes) are minute,
elongated and spindle-like in shape, with a minute rostrum anteriorly
and a long flagellum posteriorly, and very active; the female elements
(megagametes) are much larger, oblong to ovoid, and quite passive. In
_Stylorhynchus_ the difference between the conjugating gametes is not
quite so pronounced (fig. 13), the male elements being of about the
same bulk as the females, but pyriform instead of round, and
possessing a distinct flagellum; a most interesting point about this
parasite is that certain highly motile and spermatozoon-like male
gametes are formed (fig. 13), which are, however, quite sterile and
have acquired a subsidiary function. In other cases, again, the two
kinds of element exhibit either very slight differences (_Monocystis_)
or none (_Urospora_, _Gonospora_), in size and appearance, the chief
distinction being in the nuclei, those of the male elements being
smaller and chromatically denser than those of the females.
Lastly, in _Lankesteria_, _Gregarina_, _Clepsydrina_, _Diplocystis_
and _Diplodina_ complete isogamy is found, there being no apparent
difference whatever between the conjugating elements. Nevertheless,
these forms are also to be regarded as instances of binary sexuality
and not merely of exogamy; for it is practically certain that this
condition of isogamy is derived from one of typical anisogamy, through
a stage such as is seen in _Gonospora_, &c. And, similarly, just as in
all instances where the formation of differentiated gametes has been
observed, the origin of the two conjugates is from different
associates (parent-sporonts), and all the elements arising from the
same parent are of the same sex, so it is doubtless the case here.
[Illustration: FIG. 14.--Cyst of _Monocystis agilis_, the common
Gregarine of the Earthworm, showing ripe spores and absence of any
residual protoplasm in the cyst. (From Lankester.)]
The actual union is brought about or facilitated by the well-known
phenomenon termed the _danse des sporoblastes_, which is due to
various causes. In the case of highly-differentiated gametes
(_Pterocephalus_), the actively motile microgametes rush about here
and there, and seek out the female elements. In _Stylorhynchus_, Leger
has shown that the function of the sterile male gametes is to bring
about, by their vigorous movements, the _melee sexuelle_. In the forms
where the gametes are isogamous or only slightly differentiated and
(probably) not of themselves motile, other factors aid in producing
the necessary commingling. Thus in _Gregarina sp._ from the mealworm,
the unused somata or cystal residua become amoeboid and send out
processes which drive the peripherally-situated gametes round in the
cyst; in some cases where the residual soma becomes liquefied
(_Urospora_) the movements of the host are considered to be
sufficient; and lastly, in _Diplodina_, owing to the extent to which
the intertwining process is carried, if each gamete is not actually
contiguous to a suitable fellow-conjugant, a very slight movement or
mutual attraction will bring two such, when liberated, into contact.
An unusual modification of the process of sporoblast-formation and
conjugation, which occurs in _Ophryocystis_, must be mentioned. Here
encystment of two associates takes place as usual; the sporont-nucleus
of each, however, only divides twice, and one of the daughter-nuclei
resulting from each division degenerates. Hence only one
sporoblast-nucleus, representing a quarter of the original
nuclear-material, persists in each half. Around this some of the
cytoplasm condenses, the rest forming a residuum. The sporoblast or
gamete thus formed is completely isogamous and normally conjugates
with the like one from the other associate, when a single zygote
results which becomes a spore containing eight sporozoites, in the
ordinary manner. Sometimes, however, the septum between the two halves
of the cyst does not break down, in which case parthenogenesis occurs,
each sporoblast developing by itself into a small spore.
The two conjugating elements unite completely, cytoplasm with
cytoplasm and nucleus with nucleus, to form the definitive sporoblast
or zygote. The protoplasm assumes a definite outline, generally that
of an ovoid or barrel, and secretes a delicate membrane, the
ectospore. This subsequently becomes thickened, and often produced
into rims, spines or processes, giving rise to the characteristic
appearance of the Gregarine spore. Internal to the ectocyst, another,
thinner membrane, the endocyst, is also laid down. These two membranes
form the spore-wall (sporocyst). Meanwhile the contents of the spore
have been undergoing division. By successive divisions, usually
mitotic, the zygote-nucleus gives rise to eight daughter-nuclei, each
of which becomes the nucleus of a sporozoite. Next, the sporoplasm
becomes split longitudinally, around each nucleus, and thus eight
sickle-shaped (falciform) sporozoites are formed. There is usually a
certain amount of unused sporoplasm left over in the centre of the
spore, constituting the sporal residuum. It is important to note that
in all known Gregarines, with one exception, the number of sporozoites
in the spore is eight; the exception is _Selenidium_, in many ways far
from typical, where the number is half, viz. four.
[Illustration: FIG. 15.--Ripe Cyst of _Gregarina blattarum_, partially
emptied. (From Lankester.) a, Channels leading to the sporoducts; b,
Mass of spores still left in the cyst; c, Endocyst; d, The everted
sporoducts; e, Gelatinous ectocyst.]
Hitherto a variation from the general mode of spore-formation has been
considered to occur in certain Crustacean Gregarines, the
_Aggregatidae_ and the _Porosporidae_. The spores of these forms have
been regarded as gymnospores (naked), lacking the enveloping membranes
(sporocyst) of the ordinary spores, and the sporozoites, consequently,
as developed freely in the cyst. In the case of the first-named
parasites, however, what was taken for sporogony has been proved to be
really schizogony, and on other grounds these forms are, in the
present writer's opinion, preferably associated with the Coccidia
(q.v.). With regard to the _Porosporidae_, also, it is quite likely
that the gymnosporous cysts considered to belong to the Gregarine
_Porospora_ (as known in the trophic condition) have really no
connexion with it, but represent the schizogonous generation of some
other form, similar to _Aggregata_; in which case the true spores of
_Porospora_ have yet to be identified.
In the intestine of a fresh host the cysts rupture and the spores are
liberated. This is usually largely brought about by the swelling of
the residual protoplasm. Sometimes (e.g. _Gregarina_) long tubular
outgrowths, known as sporoducts (fig. 15), are developed from the
residual protoplasm, for the passage of the spores to the exterior.
Classification.
The Gregarines are extremely numerous, and include several families,
characterized, for the most part, by the form of the spores (fig. 16).
The specialized _Schizogregarinae_ are usually separated off from the
rest as a distinct sub-order.