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.