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.
| 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 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.
| 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.
The Gregarines are extremely numerous, and include several Classification. 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.