The average human adult consists of about 50 trillion _cells_—50 trillion microscopic, more or less self-contained, blobs of life. He begins life, however, as a single cell, the _fertilized ovum_.
After the fertilized ovum is formed, it divides and becomes two cells. Each daughter cell divides to produce a total of four cells, and each of those divides and so on.
There is a high degree of order and direction to those divisions. When a human fertilized ovum completes its divisions an adult human being is the inevitable result. The fertilized ovum of a giraffe will produce a giraffe, that of a fruit fly will produce a fruit fly, and so on. There are no mistakes, so it is quite clear that the fertilized ovum must carry “instructions” that guide its development in the appropriate direction.
These “instructions” are contained in the cell’s _chromosomes_, tiny structures that appear most clearly (like stubby bits of tangled spaghetti) when the cell is in the actual process of division. Each species has some characteristic number of chromosomes in its cells, and these chromosomes can be considered in pairs. Human cells, for instance, contain 23 pairs of chromosomes—46 in all.
When a cell is undergoing division (_mitosis_), the number of chromosomes is temporarily doubled, as each chromosome brings about the formation of a replica of itself. (This process is called _replication_.) As the cell divides, the chromosomes are evenly shared by the new cells in such a way that if a particular chromosome goes into one daughter cell, its replica goes into the other. In the end, each cell has a complete set of pairs of chromosomes; and the set in each cell is identical with the set in the original cell before division.
Interphase Prophase Metaphase Anaphase Telophase Interphase
[Illustration: _To study chromosomes, scientists begin with a cell that is in the process of dividing, when chromosomes are in their most visible form. Then they treat the cell with a chemical, a derivative of colchicine, to arrest the cell division at the metaphase stage (see mitosis diagram on preceding page). This brings a result like the photomicrograph above; the chromosomes are visible but still too tangled to be counted or measured. Then the cell is treated with a low-concentration salt solution, which swells the chromosomes and disperses them so they become distinct structures, as below._]