Sexual Reproduction

At least four tasks must be carried out in order to complete sexual reproduction: gametogenesis, mating, fertilization, and parental care. Within a life cycle, these biological functions can occur in close sequence, or may be separated by other activities and considerable amounts of time.

Gametogenesis

Primitively, it seems, each sex produced similar mobile gametes. The resulting condition, isogamy, is typical of sessile marine invertebrates and some algae as well as some eucaryotic organisms. Other gametic types include anisogamy and oogamy.

Types of sexual reproduction, based on gamete form. (a) isogamy: the gametes are equal in size and shape. (b) anisogamy: one gamete, conventionally termed male, is smaller than the other. (c) oogamy: one gamete, usually the larger, is nonmotile and female.

Fertilization

Fertilization is the bringing together of gametes. The parents may or may not have come together physically (that is to say mated) in order for fertilization to occur. Every route conceivable is probably used by some species as a method of carrying gametes from one individual to another. Gametes can be sent, carried, injected, inserted, and eaten in the normal course of being transferred.

One pattern which does not occur among mammals or birds, but which is common in practically every other group of organisms, is hermaphroditism. Hermaphrodites are individuals that produce both sperm and eggs. Cross fertilization occurs when two individuals of a species fertilize each other at the same time. Land snails (Helix) provide a typical example.

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