Reproduction - Sexual Reproduction in Animals

Sexual reproduction involves the fusion of male and female gametes to produce a new individual. The male gamete is the sperm, and the female gamete is the ovum (egg).

The fusion of the sperm nucleus with the egg nucleus is called fertilization, which results in the formation of a single-celled $Zygote$.

Fertilization can be of two types: Internal fertilization (occurs inside the female body, e.g., humans, cows, hens) and External fertilization (occurs outside the female body, usually in water, e.g., frogs, fish).

The zygote undergoes repeated cell division to form an $Embryo$. The stage of the embryo in which all the body parts can be identified is called a $Foetus$.

Animals are classified based on their birth process: $Viviparous$ animals give birth to young ones (e.g., humans, dogs), while $Oviparous$ animals lay eggs (e.g., frogs, butterflies).

The process of transformation from an immature larval stage to an adult through drastic changes is called $Metamorphosis$ (e.g., the life cycle of a frog: $Egg \rightarrow Tadpole \rightarrow Adult$).

Human males have a pair of testes that produce millions of sperm, each consisting of a head, a middle piece, and a tail. Human females have a pair of ovaries that produce one matured egg per month.

In humans, the male gamete ($n$) and female gamete ($n$) combine to restore the diploid number ($2n$) of chromosomes in the offspring.