ANATOMY&PHYSIOLOGY FOR NURSES

REPRODUCTION.

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 chromosomes are then attracted to the poles and lie near the new centrosomes. The chromatin of which the 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 cell resulting from mitosis contains fortysix chromosomes,  which means that during mitosis each 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 metosis.  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.