O
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A structure within the body that can be identified with the unaided eye, and which is specialised for a particular function (or functions). Organs are composed of cells and tissues. Examples of organs are: the brain, heart, lungs, stomach, and thyroid gland.
The egg cell produced by the ovary and released into the fallopian tube at the time of ovulation. The ovum is not fully matured at ovulation, and will only complete its second meiotic division if and when fertilisation occurs. It is then a haploid cell, and its female pronucleus will combine with the nuclear material from the head of the sperm to form the diploid genetic store of the new individual.