The mode of reproduction of sexual species depends on the living conditions.

Terrestrial species develop a variety of sexual rites which lead to the coupling of male and females. Very little gametes are liberated, fertilization is internal. The embryo and the foetus develop in a protected environment within the uterus.

Aquatic species liberate a large quantity of gametes into the water. Fertilization takes place exteriorly to the female body. The development of an individual is started from the larval stage. This development is "indirect"