Fertilisation

Image from http://www.pdimages.com A sperm (with tail) trying to fertilize the egg (the partially seen sphere).

Fertilisation (British English), also spelled fertilization (American English), is the process of a sperm fusing with an ovum, which eventually leads to the development of an embryo.

Animal

To deliver the sperm to female, the male inserts his external sexual organ into the opening to vagina, the passage into the female's sexual organ. (This process is a part of copulation.) Once the male ejaculates, a large number of sperms swim toward the ovum. One of them penetrates the ovum's coat, and the ovum is fertilized and the female is pregnant.

Plant

After the female part of the flower is pollinated, one pollen grain attempts to travel into the ovary by creating a path called "pollen tube." The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style[?] and curls to the bottom of the ovary, then near receptacle[?], it breaks through the ovule[?] and reaches the ovum to fertilize it. After being fertilized, the ovary start to swell and become a fruit.

See also : in vitro fertilisation, pregnancy.