For the human species to continue surviving, it is essential that mature adults are capable of producing fertile offspring, to continue the existance of the species and pass on genetic information from generation to generation.
This is done via reproduction. The following is a step by step basis of how reproduction occurs from beginning to end.
Before the initial cell can develop into a mature adult, the building blocks to create that cell must be available before fertilisation can occur. Cells called gametes are produced by mature adults via meiosis which contain half the genetic information needed to produce the final cell capable of growth.
For a short time period every month, the female gamete is readily available to be fertilised by sperm, to form a zygote.
The gametes, both containing half the genetic information needed to produce the offspring, fuse together, meaning all the genetic information required for the offspring to grow is present. For this to occur, sexual intercourse must occur in order for the semen (sperm) to be ejaculated and have the potential to fuse with the ovum.
Millions of sperm are released at the point of ejaculation, and when ejected, they 'swim' towards the female egg with their thread-like tail. This race towards the egg is fuelled by a fuel tank of ATP that provides the energy for their efforts.
After a long journey, many of the sperm will have died out in their efforts to reach the egg, though some still have to the potential to fertilise it. Each will attach itself to the ovum but only one should succeed in penetrating it. Enzymes contained in the acrosome (head) of the sperm break down the wall of the egg. When fertilised, the egg secretes various hormones to prevent it from being overwhelmed by the other millions of sperm attempting to fertilise it.
Within hours of conception the fused gametes, a zygote, undergoes cell division. The presence of a hormone called progesterone prevents further female eggs being produced. Within the first week after conception the fertilised egg travels towards the uterus, where the continued growth of the zygote will occur in the form of an embryo.
The continuation of the growing embryo is elaborated upon in the next page.