The nuclei of sperm and egg fuse inside the egg, forming the zygote. This fusion is termed fertilisation and takes place in the fallopian tube (the oviduct).

The fertilised egg or zygote begins to divide repeatedly and upon reaching a stage containing cells and a cavity called blastocoel, gets embedded in the thickened uterine wall in which many blood capillaries have formed.

In case fertilisation does not occur the egg disintegrates. The thickened wall of uterus along with capillaries breakdown leading to bleeding or menstrual flow (menstruation). In human female menstruation occurs every 28 to 30 days.

The embryo, now called foetus develops into a full fledged individual in 280 days inside the uterus and is born under the influence of a hormone from posterior pituitary, called oxytocin.

Gametes: Reproductive cells that fuse during fertilization to form a zygote which develops into an embryo. For example, Male gamete = sperm, Female gamete = egg or ovule.