(Gr. Kfipvov, nut or kernel, thus "nucleus," and yfiµos, marriage), in biology: (1) the fusion of nuclei to form a single nucleus in syngamic processes (see Reproduction); (2) the process of pairing in Infusoria, in which two migratory nuclei are interchanged and fuse with two stationary nuclei, while the cytoplasmic bodies of the two mates are in intimate temporary union.