You know that body cells divide by the process of mitosis. Mitosis produces two new cells, each containing EXACT copies of the chromosomes in the parent cell. Each new cell has a full set of chromosomes. But to produce sex cells, a different kind of cell division is needed.
A human egg an a human sperm cell each have 23 chromosomes. When an egg is joined with or fertilized by a sperm cell, a new diploid cell is formed. This new cell has 46 chromosomes, or 23 pairs of chromosomes. One set is from the mother, and the other set is from the father. The newly formed diploid cell may develop into an offspring. Meiosis (my-OH-sis) is the type of cell division that produces haploid sex cells such as eggs and sperm cells.
The development of a human embryo. Division of the fertilized egg, or ovum, begins within hours of conception. Within a week a ball of cells – a blastocyst – has developed. After the third week, the embryo has changed from a mass of cells into a recognizable shape. At four weeks, the embryo is 3 mm/0.1 in long, with a large bulge for the heart and small pits for the ears. At six weeks, the embryo is 1.5 cm/0.6 in, with a pulsating heart and ear flaps. At the eighth week, the embryo is 2.5 cm/1 in long and recognizably human, with eyelids, small fingers, and toes. From the end of the second month, the embryo is almost fully formed and further development is mainly by growth. After this stage, the embryo is termed a fetus.