A cell does not go on growing indefinitely in size but at a certain optimum point divides into two daughter cells. Further, certain cells will undergo division to replace worn-out cells or those destroyed by disease. This kind of cell division is called mitosis or karyokinesis.
Activity begins in the nucleus. The nuclear membrane disappears and the chromatin changes character and becomes long filaments called chromosomes.
The centrosome divides and the two new centrosomes move away from each other to each end of the nucleus called the poles.
The chromosomes are then attracted to the poles and lie near the new centrosomes. The chromatin of which nucleus is formed now comes to rest and two new nuclei exist. Finally the protoplasm of the cell constricts and divides and the two new cells are complete.
Each new daughter cell resulting from mitosis contains forty six chromosomes, so that during mitosis each chromosome must duplicate itself. The process of chromosomal duplication is one of the least understood of the cell’s activities.
However, mitosis is not the only kind of cell division. In the sex organs, the ovary and testis, another kind of cell division occurs called meiosis. During the formation of the sex cells, or gametes, the number of chromosomes is halved, so that the spermatozoon contains only twenty-three chromosomes and the egg-cell or ovum, twenty three.
When fertilization occurs, that is when spermatozoon and ovum fuse to form the cell (zygote) which develops into a new individual the normal chromosomal complement of forty six is restored. By this means a mixing of the hereditary determinants or genes, from male and female is achieved.