These genes, partly derived from the father and partly from the mother, decide the physical appearance of the child and the basic manner in which he or she will react to the environment. Man is derived from a single female cell, the ovum, and a single male cell, the sperm. Both cells contain a nucleus; each nucleus contains a material, chromatin, which is visible under a microscope as fine, tangled threads. When the ovum is fertilized by the sperm, the two nuclei fuse to form a single cell. The billions of cells in an adult are all ultimately derived from this single cell. Thus the original cell will transmit equal elements of both male and female to the two cells into which it divides and this process will continue through all stages of growth. Before the fertilized cell divides, the tangled filamentous chromatin coalesces to form a number of roughly X-shaped bodies, the chromosomes. These chromosomes carry the genes of inheritance.