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.
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| After Léger, 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.
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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, Léger has shown that the function of the sterile male gametes is to bring about, by their vigorous movements, the mêlée 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