E. Ray (Edwin Ray) Lankester.

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by the science of bacteriology.

The Structure and Life-history of a Typical Sporozoon. As an
example of the Sporozoa and of the characteristic features of their
life-cycle, we select for detailed description the common Monocystis
agilis, Stein 1 (Fig. 2), a Gregarine parasitic in the sperm-sacs

1 With regard to the proper name of this species, there ia a certain amount
of confusion and uncertainty, which is none the less regrettable because of so



THE SPOROZOA



'55



(vesiculae seminales) of the earthworm (Lumbricus spp.). This
species is not only very easily obtained, but is also a very typical
example of the class ; hence in describing the various phases of its
life-history it will be possible at the same time to introduce and
define the terminology to which we shall adhere in the sequel




Fio. 2.

Trophozoites of Monocystis agilis. a and fc,
young individuals showing changes of body-
fonn due to contractility, c, an older in-
dividual, still enveloped in a coat of sperma-
tozoa, (a and b after Stein, c after Lieberkiihn,
from Lankester.)



Fio. 3.

Trophozoites of Monocystis
magna, attached to the seminal
funnel of Lumbricus. a, young
individual ; b, goblet - shaped
epithelial cells of the seminal
funnel, in which the extremity
of the parasite is inserted.
(After Blitschli, from Lankes-
ter.)



for the corresponding
stages of other Sporo-

zoa. It should be understood, however, that the form
which can be selected as most typical of a group is not
necessarily the most primitive of its members. From the
type chosen we shall have to work backwards to simpler
forms, as well as forwards to more complex.

The earthworm is infested by various species or varieties
of Monocystis; according to Cue" not [13] by no less than seven
or eight species, of which four are stated to be of common
occurrence, namely, M. magna, A. Schmidt ; M. lumbrici (Henle)
( = M. agilis, Stein) ; M. pilosa, Cue"not ; and M. porrecta, A.
Schmidt. The specific distinctness of all these forms cannot
be unhesitatingly conceded, but at least two distinct species,
probably with several varieties, are generally recognised, and are
to be found in almost every worm, viz. M. magna and M. agilis.
The two species differ in size and in other specific details of
character. M. magna (Fig. 3) is the larger of the two, and
occurs attached by one extremity of the elongated body to
the epithelium of the seminal funnel, only quitting this situa-
tion at the period of conjugation, when it drops off into the
sperm-sac. M. agilis (Fig. 2) is found in the interior of the
clumps of developing spermatozoa, or floating freely in the sperm -

frequent occurrence in the zoological nomenclature of just the commonest or most
familiar forms of life, particularly amongst the Sporozoa. According to Labbe
[4] the species under consideration should be called Monocystis tenax (Dujardin) ;
according to Cue"not [13] its proper designation is M. lumbrici (Henle). We refer
to these authors for a discussion of these knotty questions, and retain here the name
most generally employed, in this country at least, for the species.




156 THE SPOROZOA



sacs. But all essential details of the life-history are quite similar in the
two species.

The earliest known stage of Monocystis agilis is a minute
protoplasmic body, with a distinct nucleus, lodged in one of
the " sperm-morulae " floating in the sperm-sac. As is well known,
each sperm-morula of the earthworm gives rise to a cluster of
spermatozoa, attached by their heads to a central residual mass
of protoplasm termed the " sporophore." The young Monocystis
is found within the sporophore and grows at its expense and at
that of the attached spermatozoa. This stage of the parasite,
during which it is absorbing nutriment from its host and growing
rapidly, may be termed the trophic stage, and each individual
parasite during this stage may be termed a trophozoite. The
parasite soon becomes elongated in one direction. It assumes
first an oval contour and becomes later more or less vermiform.
As it grows it destroys the sperm-cluster in which it is lodged,
and in later stages it is found enveloped in an adventitious coat
or fur composed of the tails of the degenerated spermatozoa,
giving the appearance of a ciliated covering, which is thrown off
in the final stages of growth (Figs. 2, c, and 4, a).

The full-grown trophozoite (Fig. 2) is still a single cell, with a
single nucleus. The body is limited by a distinct cuticle, within
which the protoplasm is differentiated into an external clear cortical
layer or ectoplasm, and an internal granular medullary layer or
endoplasm. The ectoplasm is the seat of contractility, and contains
in its deepest part a layer of fine contractile fibres, the so-called
myocyte-fibrillae. The endoplasm lodges the nucleus, and contains
numerous coarse granules representing nutriment held in reserve
for impending reproductive and developmental processes. The
nucleus is a clear spherical body, in the form of a vesicle
limited by a delicate membrane, containing fluid in which float
one or more nuclear corpuscles or karyosomes. Each karyosome is
a small globule, resembling in appearance the nucleolus of a
tissue-cell, but differing from it in containing a certain amount of
chromatin in its substance. The karyosomes usually have a
vacuolated structure. The trophozoite is actively motile, as the
specific name implies. In Monocystis the movements consist chiefly
of changes of form brought about by the contractility of the
myocyte-fibrillae, whereby the body may be bent or contracted
as a whole, or may exhibit ring -like constrictions in different
parts.

After the trophic stage, which is a period of purely vegetative
growth, the parasite enters upon the reproductive phase of its
life-history, a period in which two distinct events follow each
other; first, the formation of gametes or conjugating individuals,
which pair with one another and unite to form zygotes ; secondly,



THE SPOROZOA



157



the formation from the zygotes of the resistent spores, by which
the parasite is disseminated (see Fig. 29, p. 185).

The adult trophozoite, when it is ripe for reproduction, is
commonly known as a sporont, but may be better termed a
gametocyte, since it gives rise to the gametes. Two gametocytes
come together and become very closely apposed to form a spherical
body (Fig. 4), the two individuals remaining, however, perfectly





PlO. 4.

Association and encystation of
Monocystis magnet,, a, a couple
attached to the ciliated epithelium
of the seminal funnel of the earth-
worm. The two gregarines are
covered by a furry coat of adherent
spermatozoa, b, a couple detached
from the seminal funnel and en-
veloped by a common cyst-mem-
brane. s.f, seminal funnel ; n,
nucleus of the parasite ; c.m, double
cyst - membrane. (After Cuenot.)
a, x37; b, slightly less.



distinct from one another, each forming one hemisphere of the
common mass. This union of the two gametocytes must not be
confounded with the true conjugation : the two individuals are
merely in association they are keeping company, as it were, as
a preliminary to the formation of gametes. The two associated
gametocytes, which often exhibit a slow rotatory movement, now
become surrounded by a common envelope or cyst (Fig. 4, b, c.m),
secreted by them in two layers ; first a rigid external epicyst, then
a thin internal endocyst. 1 Meanwhile important changes are going

1 According to Cecconi [11], in M. ay His the sporonts first become encysted
singly, and two such cysts then approach each other and join together. This prob-
ably applies only to the first signs of cyst-formation, as two completely encysted
gregarines can hardly be suiliciently motile to admit of their travelling towards one
another.



158



THE SPOROZOA



D. T



dir.




fl



FIG. 5.

Formation of the segmentation-
nucleus, and its subsequent division,
in sporonts of Monocystis. a, portion
of a section through two encysted
associated sporonts, separated by
their cuticular body-walls (c.w). In
the lower half of the section is seen a
nucleus in which the karyosomes (ky)
are breaking up, while in the nuclear
sap near them a number of granules
of chromatin have appeared to form
the segmentation - nucleus (n.seg).
Outside the nuclear membrane a
patch of archoplasm (arch) has appeared. In the upper half the archoplasm has divided
to form an achromatic spindle (ac.sp), in the middle of which are seen the chromatin granules
of the segmentation -nucleus; two karyosomes are also seen, one showing a vacuolated
structure. 6, section through one sporont of an associated couple in a cyst, showing the
segmentation-nucleus in the diaster stage. The karyokinetic spindle (ac.sp) stretches across
the whole body. The chromosomes (chr) form two groups, c, section through a couple of
encysted sporonts, showing in the lower one two resting nuclei preparing for division, and the
remains of a karyosome in the cytoplasm ; in the upper one two nuclei in the diaster stage, and
two karyosomes ; ep, epicyst ; en, endocyst. d, section through one sporont of a couple showing
seven resting nuclei (n), one dividing nucleus (?uc), and two karyosomes (ky). (After Cuenot.)
a, x 1180 ; b and <l, x 500 ; c, x 790.



THE SPOROZOA 159



on within the bodies of the gametocytes. In the nucleus of each
individual the karyosomes break up and become partially dis-
solved in the nuclear sap (Fig. 5, a-d, ky). At the same time a
number of chromosomes, in the form of grains or short filaments
of chromatin, appear grouped in a clump in the nuclear sap, 1
constituting a renovated chromatic nucleus which may be termed
the segmentation-nucleus (Fig. 5, a, n.seg). At this point the
nuclear membrane disappears and the segmentation - nucleus
divides by karyokinesis, forming a nuclear spindle which
becomes elongated until it stretches across the whole body of
the gametocyte (Fig. 5, a and b, ac.sp). The two daughter-nuclei
divide again in their turn, and in this way repeated nuclear
divisions follow one another in each gametocyte ; but those in one
individual take place independently of those in the other, and
are not synchronous (Fig. 5, c). The karyosomes of the primitive
nucleus are left free in the cytoplasm and are slowly absorbed.
As the nuclei multiply, their size, and that of the karyokinetic
spindles, diminishes until it reaches a minimum (Fig. 5, b and d).
Nuclear division then ceases, and the minute nuclei travel to the
surface of the body. The cytoplasm of the gametocyte now
breaks up into a number of small masses each centred round
one of the tiny nuclei. Each of the small nucleated bodies thus
formed is commonly termed a sporoblast, but should be dis-
tinguished as a primary sporoblast, or better still, as a gamete
(Fig. 6, a). The protoplasm of the gametocyte is not entirely
used up to form the gametes, but a surplus of residual protoplasm
is left over, termed the cystal residuum (" reliquat kystal," " Rest-
korper "), which serves for the nutrition of the sporoblasts during
their further development. In the residuum are found also a
certain number of degenerated nuclei (Fig. 6, r.p, r.ri),

The next step is the conjugation of the gametes, which takes
place within the cyst. The cuticle which formed primitively the
body -wall of each gametocyte becomes dissolved, and the two
original individuals can no longer be distinguished, since the
gametes and other protoplasmic fragments derived from them
become intermingled. The gametes themselves now begin to
exhibit lively movements, the so-called " dance of the sporoblasts,"
which gradually cease as they conjugate in pairs. It is probable
that in each pair one gamete is derived from one of the two
parent gametocytes, and the other from the other, but it is by no
means certain that this is always the case. The two conjugating
gametes unite completely to form a single zygote or definitive sporoblast,

1 According to Cuenot the chromosomes are formed independently of the karyo-
somes, but it is more probable that, as in Coccidia (p. 216), their chromatin substance
is derived from a part of that which is stored up in the latter. The more recent
observations of Prowazek [25a] confirm this supposition.



i6o



THE SPOROZOA




Fio. C.



in which the two nuclei also
fuse, the grains of chromatin
being intermingled, but re-
taining their distinctness
(Fig. 6, b, *^).

Each sporoblast now
becomes a spore in the fol-
lowing way. The sporo-
blast becomes of oval form
and secretes on its surface
a tough membrane or sporo-
cyst, of a substance resem-
bling chitin (Fig. 6, c, Fig.
7, A, B). Within the
sporocyst the nucleus of
the sporoblast, or, as it
may now be termed, the
sporoplasm, divides into two,
then into four, and finally
into eight nuclei, by three
successive amitotic divi-
sions. The eight nuclei
take up an equatorial posi-
tion, and round each one
some of the protoplasm of
the spore becomes aggre-
gated, and segmented oft' as
a minute sickle-shaped germ
(Fig. 7, G) termed a spwozoite

FIQ. 6.

Formation of gametes (primary sporo
blasts) and their conjugation in Mono-
cystis, seen in sections, a, portion of
a section through a sporont showing
the gametes (g) formed at the periphery
of the body round the residual proto-
plasm (r.p). Occasionally a gamete (g 1 )
may be formed deep in the residual
protoplasm, which contains also re-
sidual nuclei (r.n). b, the gametes
havo fused in pairs to form zygotes,
in all of which the fusion of the cyto-
plasm is complete, but the nuclei are
either still separate Or 1 ) or beginning
to unite (r2) or completely fused (r3).
The residual protoplasm (r.)>) is break-
ing up into separate masses, in some
of which degenerating residual nuclei
(T.JI) are to be found; n/, cyst -en-
velope, o, the zygotes (definitive spm-o-
blasts)have begun to secrete sporooysts
(./>.<), within which the sporophism
(s;>.jp) is becoming contracted ; a lew
sporoblasts degenerate (*;/.W). Other
letters as before. (After Cuenot.)
x 1180.



THE SPOROZOA 161




("falciform body," " Sichelkeim "). The protoplasm of the sporo-
zoites is finely granular, and when they are formed, a surplus of
coarsely granular protoplasm is left over from the sporoplasm as
the sporal residuum ("reliquat sporal"). The fully-formed spore
has in Monocystis the form shown in Fig. 7, C ; it is more or less
boat-shaped, and resembles a diatom of the genus Navicella, whence
is derived the name pseudonavicella, by which Gregarine spores have
long been known. The sporocyst is slightly thickened at each

Fio. 7.

Development of the spore of Mono-
cystis, A, oval sporoblast with single
nucleus (a). B, the sporoblast has
secreted the sporocyst at its surface,
and the sporoplasm within it has
become contracted and diminished
in volume. C, ripe spore with eight
sporozoites and residual protoplasm
(6). D, diagrammatic cross-section to
show the arrangement of the sporo-
zoites round the central residual pro-
toplasm. (After Biitschli, from
Lankester.)

pole, and within this very resistent and impervious envelope the
eight sporozoites are packed lengthways round the centrally placed
sporal residuum. During the formation of the spores the cystal
residuum is slowly absorbed, and the ripe cyst contains only a
great number of the pseudonavicellae, not arranged in any definite
pattern (Fig. 8).

The above account of the conjugation and spore - formation is that
given recently by Cuenot, whose researches confirm the discoveries of
Siedlecki with regard to an allied form Lankesteria ascidiae (Lank.), and
are in harmony with the still more recent account given by Leger [23]
for Stylorhynchus. Cuenot's description of the spore-formation and the
events antecedent to it is confirmed in all essential details by Cecconi [11]
and Prowazek [2 5 a]. Previous to Cuenot the reproduction of Monocystis
had only been studied by Wolters [29], whose description of the process
is very different. According to Wolters the association of the two full-
grown trophozoites or sporonts within the cyst is a true conjugation,
similar in its details to that known in Actinophrys from the researches
of Schaudinn. Wolters describes the nucleus of each sporont as dividing
mitotically to form two nuclei, one of which is given off in a polar body,
while the other remains as a pronucleus. The two nuclei are then stated
to travel towards the septum formed by the apposition of the cuticular
body-walls of the two sporonts, and at one point the septum becomes
dissolved, permitting the fusion of the pronuclei into a single nucleus.
After a time the fusion -nucleus divides into two nuclei, which then
rapidly divide up to form numerous small nuclei, round which the proto-
plasm of the sporonts becomes segmented to form the sporoblasts. From
the sporoblasts the spores are formed as above described.

It is unfortunate that these statements of Wolters, which seem to be

ii



162



THE SPOROZOA



totally erroneous, have remained uncontradicted for ten years, during
which time they have got into numerous text -books and have been
generally accepted and taught

The spores of Monocystis do not appear to be able to develop
further in the earthworm, but require to be transferred to a
fresh host before they can germinate. How the infection is
effected has not yet been ascertained in the case of the type




Fio. 8.

Ripe cyst of Monbcystis, showing the numerous spores (pseudonavicellae) scattered within
the cyst, without any cystal residuum present. (From Lankester.)

that has been selected for description, and the course of events
can only be conjectured by analogy from what is known to
take place in other Sporozoa. It is highly probable that the
spores pass to the exterior and are scattered broadcast in the
earth, and that they are then swallowed accidentally by an earth-
worm with its food, and so pass into its digestive tract. The
action of the digestive juices upon the spores has the effect of
causing the sporocysts to burst open, setting free the sporozoites,
which are actively motile and possess the power of boring their
way through cells and tissues. In this way the sporozoites prob-
ably traverse the wall of the earthworm's intestine and reach the
reproductive organs, where each one attacks a sperm-mother-cell



THE SPOROZOA 163



and there develops into the minute trophozoite which is formed
later within the sperm-morula. With this stage the life-cycle has
come round again to the point at which the description of it was
commenced.

A few points with regard to the life-cycle require brief further dis-
cussion. It has been suggested that the spores may sometimes germinate
in the host in which they are formed, and so increase the numbers of the
parasite within it. But the improbability of this occurring is very
great, as was pointed out by Butschli, in view of the relatively small
number of parasites in the trophic stage which are met with, as com-
pared with the vast number of spores. Thus in a given earthworm there
will be found in the sperm -sacs perhaps a dozen trophozoites and as
many ripe cysts. Each of the latter contains, however, at a low esti-
mate about fifty spores, and each spore eight sporozoites. A single cyst
contains, therefore, about four hundred individuals, more or less, and if
it were a frequent occurrence for the spores to germinate in the same host,
the number of trophozoites in each earthworm might be expected to be
vastly greater than is usually the case.

Another question which may be raised is whether the Monocystis has
any method of multiplication during the trophic phase, that is to say, in
the period from sporozoite to sporont. It has sometimes been stated that
the trophozoites multiply by division during the earlier stages of growth.
From what is known of other Sporozoa, there is nothing inherently
improbable in this view, but it has not been proved satisfactorily
that such multiplication can take place in Monocystis, and the above-
mentioned paucity of the trophozoites is an argument against its
occurrence.

With regard to the passage of the spores to the exterior, precise
information is lacking as to how this is effected. In Sporozoa generally
we find one of two conditions. In some cases the spores are produced
in a position where they can leave the body by natural channels, as in
the numerous instances of sporozoan parasites lodged in the digestive
tract, when the cysts and spores are cast out with the faeces. In other
cases the spores cannot pass out by natural channels, and are set free
either by provoking suppuration or other organic disturbance, or by
the death and break-up of the host. In the case of the Monocystis of
the earthworm, the spores could only be discharged from the body, in the
ordinary course of events, by passing out of the sperm-sac with the sperm
at copulation. They would then be transferred to the spermathecae or
receptacula semiuis of another worm, and would pass ultimately into" the
cocoon in which the eggs are laid ; but there is no record of their occur-
rence in either of these situations. It seems more probable that spores
are set free by the dissolution of their host. Very possibly birds or
some other of the numerous creatures which prey upon worms are the
agents by which the dissemination is effected. If a bird swallowed an
earthworm containing spores of Monocystis, from which very few worms
are free, the spores would probably pass unaltered thorough the bird's
digestive tract. Uninjured spores of Gregarines have been observed by



1 64 THE SPOROZOA



L. Pfeiffer in the intestines and faeces of various birds. 1 A parallel case
is that of the Coccidium infecting the centipede Lithobius, the spores of
which, if swallowed by another animal, such as a wood-louse, pass unaltered
through it (see p. 221). If this suggestion be correct, it is easy to under-
stand that any worm-eating bird would be continually scattering spores
of Monocystis on the ground, where they would wash down into the soil
and be swallowed very easily by worms again. There is, however, no
direct evidence bearing upon the mode of dissemination of the spores,
and the above suggestion must be regarded merely as a more or less
probable surmise.

When the spores have reached the digestive tract of their new host,
and the sporozoites have been liberated there, the question arises how
they reach the sperm-sacs. This problem, however difficult to solve, is
by no means one peculiar to the Monocystis of the earthworm. In many
other Sporozoa we have instances of parasites affecting some particular
organ, which invade the body in the first place from the digestive tract.
It must be assumed that the sporozoites have in some way the power of
selecting the particular organ they affect, and of migrating through the
body of the host in order to reach their specific habitat. Probably they
make use of vascular or lymphatic channels in order to arrive at their
destination.

General Characters of the Sporozoa. From the above account
of Monocystis it is seen that the life-history of a typical Sporozoon
is a single cycle, which may be summed up in the following
way : 2

Sporozoite - >-Trophozoite-^-Gametocyte (Sporont) x n Gametes \ ,
Sporozoite->-Trophozoite->-Gametocyte (Sporont) x n Gametes / T

=n Zygotes (Sporoblasts)-^-?i Spores x Sn Sporozoites.

The life-cycle may further be divided into three main periods.

First, the period of growth, during which the minute sporo-
zoite grows by absorption of nutriment from the host into the
sporont.

Secondly, the period of proliferation, accompanied by conjuga-
tion, and resulting in the formation of a large number of germs,
destined to spread the species.

Thirdly, the period of rest, during which the parasitic germs
pass out from the host into the outer world, to effect, if fortune
favour them, the passive infection of a new host.

In Sporozoa, considered generally, the life-history is similar in
the main to that described above, but exhibits, in different forms,
variations of every kind, in the direction either of greater or
of less complexity. The deviations from the selected type may

1 Fide Wasielewski [7], p. 26.

3 In this and in all subsequent formulae of sporozoan life-histories au arrow is
used to mean "becomes" or "grows into" ; the sign x to indicate a distinct cell-
generation, a multiplication of individuals of any kind ; and a bracket with the
sign + to denote the occurrence of zygosis or true conjugation Mid fusion of gametes.



THE SPOROZOA 165



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