The process of conjugation occurs, in the same manner as in _Noctiluca_, in a freshwater Rhizopod, the well-known Sun-animalcule, _Actinophrys sol_ (Fig. 84), but in this case complete fusion of the two nuclei takes place (Fig. 84, _V_) before the formation of the division-spindle (_VI_, _sp_), which, with the simultaneous division of the cell-body, gives rise to two new individuals. The process in this case is especially interesting, because Schaudinn has succeeded in observing a maturation division (_III_, _Rsp_, directive spindle) as well as in demonstrating polar bodies (_IV_, _Rk_). Thus the analogy with the process of fertilization in the Metazoa and the Metaphyta is almost complete. But that the conjugation of unicellular organisms, like the fertilization of multicellular organisms, is essentially a matter of nuclear conjugation is shown more distinctly still by the ciliated Infusorians, the most highly organized of the Protozoa. [Illustration: FIG. 84. Conjugation and polar body formation in the Sun-animalcule, _Actinophrys sol_, after Schaudinn. _I_, two free-swimming conjugated individuals, which in _II_ have become surrounded by a transparent gelatinous cyst. _III_, formation of the directive spindles (_RSp_). _IV_, the polar bodies are formed (_RK_); _K_, the two sex-nuclei. _V_, these are fused to form the conjugation-nucleus (_K_). _VI_, the conjugation-nucleus is transformed into the division-spindle; the polar bodies (_RK_) have penetrated the internal cyst-wall, and are in process of degeneration.] Here there is usually no complete union of the cell-bodies of the two animals, but only an adhering of the apposed surfaces. In the relatively large _Paramœcium caudatum_ the process of conjugation is very exactly known through the beautiful investigations of Maupas and R. Hertwig. In this case the mouth-surfaces of the two animals come together and unite over a short area, and then the two animals swim about together in this conjugated state. During this time very remarkable changes take place in their nuclei.