For sexual reproduction, both male and female gametes have to be united. Sexual reproduction in animals can be observed in members of all the phyla. In protozoans, we can take an examp0le of an endoparasite, Monocystis. This parasite lives in one of the reproductive organs, the seminal vesicle of an earthworm. Two members of Monocystis which come together are called gametocytes.
Each member produces gametes by division. The gametes of these two individuals are similar-looking and hence are called isogametes. In Plasmodium, however, the two gametes involved in sexual reproduction are of different size. Macrogametocyte, which is larger, produces the rounded ovum, and the microgrametocyte, which is smaller, produces fusiform sperms.
In multicellular animals, some are hermaphrodites, other are ubnisexuals. Earthworm (pheretima) is a good example of hermaphrodite in which both male and female reproductive organs are housed in one member. Only the occurrence of testis and ovary in one individual is enough to call it hermaphrodite. Some species of hydra are hermaphrodites. In both earthworm and hydra, testis matures earlier than the ovary, with the that the sperms produced by an individual can not fertilize the ova of the same individual as their ovaries take longer to mature.
This condition, met with in some animals whereby the testis matures earlier than the ovary, is called protandrous condition. The idea of protandry is to prevent self-fertilization. In some hermaphrodites, however, the ovary matures earlier. This is called proitogynous condition.
In the ovum is fertilized outside the body of the female, this is referred to as external fertilization. Frog is a good example where this can be observed. The female frog discharges the eggs outside in water. The male which mounts on the back of the female at this time, also happens to discharges the sperms. The latter fertilizes the eggs in water.
In a large number of animals, however, the fertilization of ova takes place inside the body of the female which produces them. This is known as internal fertilization. This can be commonly observed in insects, spiders, reptiles, birds, and mammals. In reptiles and birds, fertilization is internal, but the fertilized eggs are covered by calcareous shell. These eggs are released outside and embryonic developments takes place outside the body of the mother inside the shell.
These animals are, therefore, called oviparous (ovi=egg; parous=to produce). But in some animals, for example in mammals, fertilization is internal, and the embryonic development too is completed inside the body of the mother, who, after nurturing, gives birth to the young one. This condition is called viviparous (vivi=live).