Sexual Processes

SEXUAL PROCESSES usually consist in the formation of special reproductive cells, gametes, which subsequently unite via fertilization into a zygote (see). Sexual processes are widely distributed in both the animal and plant worlds. They are absent or have not been observed only in certain single-celled organisms (many bacteria, most flagellates, and some others). In multicellular organisms, only a portion of the body cells, the germ cells, go on to form gametes, whereas the remaining somatic cells are sterile; in single-celled organisms, the entire organism as a whole forms gametes, which is usually preceded by a series of special progamic divisions. The resulting gametes (see) in the simplest cases are of equal size and structure (isogametes), but in all multicellular organisms they differentiate into male and female anisogametes, i.e., spermatozoa and eggs. In lower organisms (algae), anisogamy has a relative character, such that the very same gametes in different combinations exhibit either a female or a male character (see Sex). Gametes uniting during fertilization usually originate from different individuals of the animal; however, in certain Protozoa, the fusion of closely related sexual elements takes place (see Pedogamy, Autogamy). In hermaphroditic multicellular organisms, self-fertilization is sometimes observed, i.e., sexual processes between spermatozoa and eggs of the same individual. Sexual processes are ordinarily accompanied by the complete fusion of the gametes and their nuclei (see Copulation), but in certain Protozoa (ciliates) they are limited to the temporary union of two cell-individuals, during which they exchange parts of their nuclear apparatus (see Conjugation). A departure from the typical sexual process is parthenogenesis (see), i.e., the development of a female gamete without its fusion with a male gamete. The essence of the sexual process lies in the alteration and renewal of the nuclear substance of the zygote or parthenogenetic egg. In connection with this, fertilization is usually preceded by special phenomena of gamete maturation (see Fertilization, Ovulation). As a result of changes in the nuclear substance of the zygote, the latter acquires new combinations of properties and hereditary traits compared to the organisms that gave rise to it. For the most part, sexual processes represent the beginning of sexual reproduction, but these two phenomena cannot be equated. Thus, in many Protozoa, the reproduction following the sexual process differs in no essential way from asexual reproduction. In some multicellular organisms (salps), true reproduction occurs exclusively through asexual means, whereas sexual processes lead to the formation of only a single embryo in each sexual individual, the birth of which is accompanied by the death of the mother; in this case, sexual processes do not lead to reproduction, i.e., an increase in the number of individuals.

V. Dogel.