(1) In animals, the fertilization of an ovum by several spermatozoa. Two types of polyspermy are distinguished: physiological and pathological.

Physiological polyspermy is characteristic of several groups of animals with internal insemination: spiders, insects, fishes of the class Chondrichthyes, amphibians of the order Caudata, reptiles, and birds. In such animals, the number of spermatozoa penetrating the ova varies from one or two to 12 in insects and numbers several dozen in chordates. In the ooplasm all the spermatozoa change similarly: their heads become spermatic nuclei. The nucleus nearest to the female pronucleus merges with it and forms a syncaryon, which proceeds to the stage of cleavage. The remaining spermatic nuclei also begin the process of mitosis, but in insects, caudates, and reptiles the process is blocked and the mitotic figures are soon resorbed.

Among birds, the spermatic nuclei divide repeatedly and degenerate at the stage of eight to 16 blastomeres. Among Chondrichthyes, the spermatic nuclei divide many times and are gradually ousted by the segmentation nuclei, the offspring of the syncaryon, beyond the boundaries of the embryonic disk; thus, they do not participate in the formation of the body of the embryo. The suppression of mitosis and the resorption of all the spermatic nuclei except one are caused by changes in the properties of the cytoplasm of the fertilized ovum, whose nature is as yet unknown.

Pathological polyspermy occurs among physiologically mono-spermous animals having external and in some groups, including mammals, internal insemination. When there is an excessive concentration of spermatozoa or when the ova are in poor physiological condition, the mechanisms ensuring normal monospermy are insufficiently effective and each ovum may be penetrated by several spermatozoa. These spermatozoa take part in the embryo’s development and cause profound disturbances; the embryo soon dies.

(2) In plants, the penetration of more than one spermatozoon, usually two, into the ovum and the secondary nucleus of the embryonic sac, irrespective of the subsequent fate of the spermatozoa.