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| Sexual Reproduction in Animals |
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| Sexual reproduction is seen in nearly all animals. In animals reproduction also involves production of gametes that are haploid cells. In unicellular organisms like the protozoans, the gamete-producing individuals are called gametocytes. The gametes may be similar - isogametes as in Monocystis or dissimilar - heterogametes as in Plasmodium. In the latter the gametocytes are: |
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 Macrogametocyte (female) |
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Microgametocyte (male) |
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| In multicellular organisms, the gametes are produced by the reproductive organs. The male gametes called the sperms are produced by the male reproductive part and the female gametes called the eggs are produced by the female reproductive part. Production of sperms is called spermatogenesis and production of eggs is called oogenesis. In most cases the gametes are produced by different individuals. However, in some cases like that of earthworm, the same individual produces both the types of gametes. It is called a hermaphrodite. However, fertilisation is between the gametes of different individuals. |
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| Most of the animals are unisexual with an individual only producing either male or female gametes. The male gamete of one individual fertilizes with the female gamete of another individual to produce the zygote. The fertilisation may be external or internal. |
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| External fertilisation takes place in animals like the fish and frog where the eggs are released from the body of the females into the water outside. |
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| Frog, Ovulation, Accumulation of Eggs in Uterus, Spawning and External fertilisation |
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| These eggs are then fertilised by the sperms produced by the male species. The fishes and frogs are oviparous, that is they lay eggs. |
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| Organisms like birds, insects, reptiles are also oviparous. However, in these organisms the fertilisation is internal. Internal fertilisation occurs in mammals also. In internal fertilisation the sperms are released into the body of the females during copulation. The resulting zygote develops into a young one. Thus mammals are viviparous, that is, they give birth to young ones. |
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