The third mode of reproduction, by spore formation, does not essentially differ from that by multiple fission. It consists in the breaking up of the organisms into a number (usually very considerable) of portions; each of which eventually develops into an organism like the parent form. All gradations between a simultaneous division of the organism into such spores and simple multiple fission are to be found, but this process of reproduction may be sometimes distinguished from that by such fission by the fact that the two processes may coexist in a single form, _e.g._ the biflagellate monad of Drysdale and Dallinger. In the majority of cases the spores produced differ at first from the parent organism not only in size but in other points, such as the possession of a flagellum, etc. They may even be without a nucleus when the parent organism is nucleated, as in the Gregarinidæ. The encystment, which in many cases precedes reproduction by any of the above processes, and more especially by spores, is not an essential condition of their occurrence; and is probably in the first instance a protective arrangement which has become secondarily adapted to and connected with reproduction. As has been already stated, all the above modes of reproduction take place in some of the Protozoa without any anterior process which can be regarded as of a sexual nature; but very often they are preceded by the temporary or permanent fusion of two or more individuals, such fusion being known as conjugation. In most cases reproduction by spores is the consequence of conjugation, but in the Infusoria etc. where the fusion at conjugation is temporary (except Vorticella), there is probably merely a renewed activity--a rejuvenescence--which most likely results in active fission or budding. In the Gregarinidæ reproduction by spores usually follows conjugation, but may also take place without it. In some Flagellata reproduction by spores follows the conjugation of two individuals in a different stage of development. Thus in the springing Monad, described by Drysdale and Dallinger, a form produced by the fission of a monad in an amoeboid condition fuses with an ordinary monad to produce an individual, which then breaks up into spores. Another instance of the fusion of dissimilar individuals is afforded by Vorticella, where a free-swimming individual conjugates and is permanently united with a fixed one (Engelmann, Bütschli). Conjugation often consists in the fusion of more than two individuals. In conjugation where the fusion is permanent, the nuclei of the conjugating forms usually unite before the product breaks up into spores and where temporary fusion occurs in the Infusoria a division of the paranuclei and often of the nuclei takes place, followed by the ejection of parts of them, and a reproduction of new paranuclei and nuclei from the remainder of the original structures.