This is the method of certain protozoans through which the mixing of the genes is achieved. As worked out in certain species of Paramecium, the process is as follows. Several different mating types exist and any one can conjugate with any of the other mating types, but not with its own.
The two animals from two different mating types come together at their oral grooves and form a protoplasmic connection between them. Meanwhile the small micronucleus of each divides meiotic ally to form four haploid micro- nuclei.
Out of these four micronuclei three become disintegrated and the remaining one undergoes a mitotic division to form two micro- nuclei. One of these from each animal migrates across the protoplasmic connection and unites with the micronucleus remaining in the opposite conjugant. The animals now separate and the fusion nucleus undergoes divisions accompanied with later cellular divisions.
Image Source : scientificcomputing.com
External fertilization:
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Most animals produce sperms and eggs, but in many water species these gametes unite in water; there is no union between opposite sexes. This is called external fertilization. In almost all the aquatic animals the fertilization is external.
Internal fertilization:
In all land animals the fertilization is internal. To make the fertilization successful the two opposite sexes of the same species undergo copulation, whereby the sperms can be transferred to the body of the female where fertilization takes place. Many water animals also have this process although it is not a rule.
Place of embryonic development:
Those animals which lay eggs are said to have oviparous reproduction. In such cases the major part of embryonic development takes place outside the female body, even though fertilization has been internal. Those animals which give birth directly to the fully developed young ones are said to have viviparous reproduction.
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For example, mammals have a small egg with comparatively little yolk and the embryo develops within the mother’s body, deriving nourishment from her body in the process. There is third type of reproduction what is known as ovoviviparous reproduction where there is a large egg which furnishes food for the developing embryo but there is internal fertilization and the egg remains in the female until it hatches. Thus, the young are born fully developed and active, but they have not derived nourishment from their mother during embryonic development.