Reproduction by fission is not rare, and in some cases (_Acanthocystis_) the cell becomes multinuclear, and buds off 1-nucleate cells. In such cases the buds at first lack a centrosome, and a new one is formed first in the nucleus, and passes out into the cytoplasm. These buds become 2-flagellate before settling down. In _Clathrulina_ the formation of 2-flagellate zoospores has long been known (Fig. 20, 3). In _Actinosphaerium_ (Figs. 19, 21), a large species, differing from _Actinophrys_ only in the presence of numerous nuclei in its endoplasm, a peculiar process, which we have characterised as _endogamy_, results in the formation of resting spores. The animal retracts its rays and encysts; and the number of nuclei is much reduced by their mutual fusion, or by the solution of many of them, or by a combination of the two processes. The body then breaks up into cells with a single nucleus, and each of these surrounds itself with a wall to form a cyst of the second order. {74}[Illustration: FIG. 20.—Various forms of Heliozoa. In 3, _a_ is the entire animal and _b_ the flagellula; _c.vac_, contractile vacuole; _g_, gelatinous investment; _nu_, nucleus; _psd_, pseudopodia; _sk_, siliceous skeleton; _sp_, spicules. (From Parker and Haswell, after other authors.)] Each of these divides, and the two sister cells then conjugate after the same fashion as in _Actinophrys_, but the nuclear divisions to form the coupling nucleus are two in number, _i.e._ the nucleus divides into two, one of which goes to the surface as the first polar body, and the sister of this again divides to form a second polar body (which also passes to {75}the surface) and a pairing nucleus.[84] The two cells then fuse completely, and surround themselves with a second gelatinous cyst wall, separated from the outer one by a layer of siliceous spicules. The nucleus appears to divide at least twice before the young creep out, to divide immediately into as many _Actinophrys_-like cells as there were nuclei; then each of these multiplies its nuclei, to become apocytial like the adult form. [Illustration: FIG. 21.—Diagram illustrating the conjugation of _Actinosphaerium_. 1, Original cell; 2, nucleus divides to form two, N_{2}N_{2}; 3, each nucleus again divides to form two, N_{3} and _n__{3}, the latter passing out with a little cytoplasm as an abortive cell; 4, repetition of the same process as in 3; 5, the two nuclei N_{4} have fused in syngamy to form the zygote nucleus N_{z}.]