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Reproduction existed long before sex; asexual reproduction may have predated sexual reproduction by 1.5 billion years or more. Although sexual reproduction is the primary form of reproduction in animals, the process of meiosis which is the basis for sex was essentially complete in unicellular protists. Although animals would make sexual reproduction more complex (by uniting meiotic cells to form gonads, developing tubes through which gametes could travel, adapting the process to occur inside the female body in many groups, and recruiting the nervous system to involve sexual drives and courtship behaviors), the cellular mechanisms of sexual reproduction have changed little since the evolution of the early eukaryotes.
In sponges, sexual reproduction occurs without
gonads—sex cells may originate from anywhere any part of the animal (Beklemishev, vol. 2). In
sponges, ova retain primitive characteristics in that they move like amoeba
and may perform phagocytosis (Barrington, p.
386). Some features of the gametes
of these most primitive animals are retained in later groups. For example, after the process of spermatogenesis
which generates haploid spermatids, a second
process called spermiogenesis occurs to modify
the shape of the male gamete as in higher animals, although no acrosome is formed on the sperm. (
The simplest animals, sponges and cnidarians, lack follicular cells which surround the ova. In some flatworms the ovary is compacted and divided into layers so that oocytes may be surrounded by follicular cells. In hemichordates and urochordates, follicle cells surround oocytes unlike the condition in males which possess no such cells. In both hemichordates and urochordates, oocytes are released from the ovary before meiosis is complete. Similar to the condition in vertebrates (including humans), an ova is technically not produced until fertilization (and thus a female which has not conceived has technically never produced a female gamete). (Benito, form Harrison 1997, p. 93; Burighel, from Harrison, 1997, p. 282). In the most primitive flatworms, eggs lack yolk. (Hickman, Beklemishev, vol. 2) while in others, entolecithal egg production occurs (Dougherty).
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