Gametes

GAMETES, reproductive cellular elements, or individuals, that unite during fertilization, or during conjugation, copulation, and similar processes in animal and plant organisms. In multicellular beings (animals, higher plants), gametes are specialized cells—spermatozoa and eggs; in lower animals and plants (Protozoa and Protophyta), the character of gametes differs depending on the morphological features of the sexual process. A) In copulation—the complete and final fusion of two whole individuals, which is observed, for example, in sun animalcules (Actinophrys) and in some flagellates (Scytomonas; see figure), the gametes correspond in morphological terms to the whole individual, and the process of fusion of such gametes is called hologamy. B) In conjugation—the temporary connection of the bodies of two individuals, typical for ciliates—a mutual exchange and fusion of parts of the micronucleus occurs. Here, the gametes are precisely these fusing micronuclei. In some cases, these micronuclei are clearly differentiated in sexual terms even from a morphological standpoint (see figure f), but for the most part, they are distinguishable only physiologically. C) Some

Gametes: a-Scytomonas subtilis (flagellate)—hologamous copulation; b-Polystomella crispa (rhizopod)—vegetative form forming flagellated isogametes; b'-copulation of isogametes; c-Stylorhynchus longicolis (gregarine)—copulation of anisogametes; d-Eimeria Schubergi (coccidium)—copulation of anisogametes; e-Plasmodium vivax—copulation of anisogametes; f-Cycloposthium bipalmatum (ciliate)—male and female micronuclei correspond to anisogametes. Protozoa and Protophyta form small sexual individuals—gametes (see figure b). If these gametes are morphologically indistinguishable, they are called isogametes; otherwise, they are called anisogametes. Distinguishing male and female gametes is possible only in cases of clear anisogamy, which is observed in its most perfect form during the formation of reproductive cells in higher organisms, as well as in Protozoa and Protophyta in those cases where their gametes morphologically resemble an egg or a spermatozoon, as, for example, the gametes of various sporozoans (Sporozoa), including the malaria plasmodium (see figure c, d, e). Sometimes morphological data do not allow for the detection of sexual differences between gametes; in these cases, the latter may be revealed by physiological signs—more mobile and active gametes are considered male, and passive ones are considered female. The process of gamete formation is called gametogony. Gametes are formed from gametocytes through the reduction division of the latter's nucleus. Thus, oocytes and spermatocytes of higher organisms are, in essence, gametocytes. Gametocytes for the most part also differ in sexual terms—female gametocytes are richer in nutritive inclusions in the plasma, their nucleus has a looser structure (poorer in chromatin), and has a relatively smaller volume than that of male gametocytes.