Encyclopaedia Britannica, 11th Edition, "Greek Law" to "Ground-Squirrel" / Volume 12, Slice 5

by Various

From Wasielewski, after A. Schneider.

Fig. 10.—Schizogony in Ophryocystis francisci. a, Rosette of small individuals, produced from a schizont which has just divided; b, A later stage, the daughter-individuals about to separate and assuming the characters of the adult.

From Wasielewski, after Léger.

Fig. 11.—Eirmocystis spp. a, b, Associations of two and three Gregarines; c, Chain of five parasites; p, Primite; s, Satellites.

A sexual multiplication (schizogony) is only known certainly to occur in a few cases, one being in a Monocystid form, a species of Gonospora, which is for a long time intracellular (Caullery and Mesnil [4]), the rest among the Schizogregarinae, so Life-history. named for this reason, in which schizogonous fission takes place regularly during the free, trophic condition. Usually, the body divides up, by a process of multiple fission (fig. 10), into a few (up to eight) daughter-individuals; but in a new genus (Eleutheroschizon), Brasil (3) finds that a great number of little merozoites are formed, and a large amount of vacuolated cytoplasm is left over unused.

In the vast majority of Gregarines, however, the life-cycle is limited to gametogony and sporogony. A very general, if not indeed universal, prelude to gametogony is the characteristic and important feature of the order, known as association, the biological significance of which has only lately been fully brought out (see H. M. Woodcock [31]). In normal association, two individuals which are to be regarded as of opposite sex, come into close contact with each other and remain thus attached. The manner in which the parasites join varies in different forms; the association may be end-to-end (terminal), either by like or by unlike poles, or it may be side-to-side (lateral) (fig. 12). The couple (syzygy) thus formed may proceed forthwith to encystment and sporoblast-formation (Lankesteria, Monocystis), or may continue in the trophic phase for some time longer (Gregarina). In one or two instances (Zygocystis), association occurs as soon as the trophozoites become adult. This leads on to the interesting phenomenon of precocious association (neogamy), found in non-motile, coelomic Gregarines (e.g. Cystobia, Diplodina and Diplocystis), in which the parasitism is most advanced. Woodcock (loc. cit.) has described and compared the different methods adopted to ensure a permanent union, and the degree of neogamy attained, in these forms. Here it must suffice to say that, in the extreme condition (seen, for instance, in Diplodina minchinii) the union takes place very early in the life-history, between individuals which are little more than sporozoites, and is of a most intimate character, the actual cytoplasm of the two associates joining. In such cases, there is absolutely nothing to indicate the “double” nature of the growing trophozoite, but the presence of the two nuclei which remain quite distinct.

There can be little doubt that, in the great majority, if not in all Gregarines, association is necessary for subsequent sporulation to take place; i.e. that the cytotactic attraction imparts a developmental stimulus to both partners, which is requisite for the formation of primary sporoblasts (gametes). This association is usually permanent; but in one or two cases (perhaps Gonospora sp.) temporary association may suffice. While association has fundamentally a reproductive (sexual) significance, in some cases, this function may be delayed or, as it were, temporarily suspended, the cytotactic attraction serving meanwhile a subsidiary purpose in trophic life. Thus, probably, are to be explained the curious multiple associations and long chains of Gregarines (fig. 11) sometimes met with (e.g. Eirmocystis, Clepsydrina).

Encystment is nearly always double, i.e. of an associated couple. Solitary encystment has been described, but whether successful independent sporulation results, is uncertain; if it does, the encystment in such cases is, in all probability, only after prior (temporary) association. In the case of free parasites, a well-developed cyst is secreted by the syzygy, which rotates and gradually becomes spherical. A thick, at first gelatinous, outer cyst-membrane (ectocyst) is laid down, and then a thin, but firm internal one (endocyst). The cyst once formed, further development is quite independent of the host, and, in fact, often proceeds outside it. In certain coelomic Gregarines, on the other hand, which remain in very close relation with the host’s tissues, little or nothing of an encystment-process on the part of the parasites is recognizable, the cyst-wall being formed by an enclosing layer of the host (Diplodina).

From Wasielewski, after Léger.

Fig. 12.—Associations of Gonospora sparsa.

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.

After Léger, from Lankester’s Treatise on Zoology.
Fig. 13.—Development of the Gametes and Conjugation inStylorhynchus longicollis.
a, Undifferentiated gamete,attached to body of parent-individual. b-d, Stages in development ofmotile male gamete. e, Mature female gamete.	f, g, Stages in conjugation andnuclear union of the twoelements. h, Zygote (copula). i, Spore, still with singlenucleus and undividedsporoplasm.