Reproduction in its simplest form is, then, simply the division of one
cell into two parts, each of which develops into a replica of the
original. Division is also the first stage in reproduction in the most
complicated animal bodies. To get an idea of what takes place in such a
division we must remember that a cell consists of three distinct parts:
(a) the protoplasm or cytoplasm, (b) the nucleus, and (c) a small body
known as the centrosome which need not be discussed here.

When a cell division takes place, the nucleus breaks up into a number of
thread-like portions which are known as chromosomes. There are supposed
to be 24 pairs, or 48, in the human cell. All the evidence indicates
that these chromosomes carry the "factors" in inheritance which produces
the characters or characteristics of the individual body.

In mitosis or ordinary cell division, these chromosomes split
lengthwise, so that the new cells always have the same number as the
original one. When the germ-cells of the male and female make the
division which marks the first step in reproduction, however, the
process is different. Half the chromatin material passes into each of
the two cells formed. This is called _maturation_, or the maturation
division, and the new cells have only half the original number of
chromosomes. Each of these divides again by mitosis (the chromosomes
splitting lengthwise), the half or haploid number remaining. The result
is the _gametes_ (literally "marrying cells"--from the Greek _gamé_,
signifying marriage). Those from the male are called sperms or
spermatozoa and those from the female eggs or ova. (The divisions to
form ova present certain complications which need not be taken up in
detail here.) Of the 24 chromosomes in each sperm or egg we are here
concerned with only one, known as the sex chromosome because, in
addition to transmitting other characteristics, it determines the sex of