many countries. The list of their names is long, and I shall not
even attempt to repeat it. The chromosomes come together in pairs (fig. 49
a). Each maternal chromosome mates with a paternal chromosome of the same
kind.

[Illustration: FIG. 49. In the upper row of the diagram a typical process
of nuclear division, such as takes place in the early germ cells or in the
body cells. In the lower row the separation of the chromosomes that have
paired. This sort of separation takes place at one of the two reduction
divisions.]

Then follow two rapid divisions (fig. 49 f, g and 50 and 51). At one of the
divisions the double chromosomes separate so that each resulting cell comes
to contain some maternal and some paternal chromosomes, i.e. one or the
other member of each pair. At the other division each chromosome simply
splits as in ordinary cell division.

[Illustration: FIG. 50. The two maturation divisions of the sperm cell.
Four sperms result, each with half (haploid) the full number (diploid) of
chromosomes.]

The upshot of the process is that the ripe eggs (fig. 51) and the ripe
spermatozoa (fig. 50) come to contain only half the total number of
chromosomes.

[Illustration: FIG. 51. The two maturation divisions of the egg. The
divisions are unequal, so that two small polar bodies are formed one of
these subsequently divides. The three polar bodies and the egg are
comparable to the four sperms.]

When the eggs are fertilized the whole number of chromosomes is restored
again.

THE MECHANISM OF MENDELIAN HEREDITY DISCOVERED IN THE BEHAVIOR OF THE
CHROMOSOMES

If the factors in heredity are carried in the chromosomes and if the
chromosomes are definite structures, we should anticipate that there should
be as many _groups_ of characters as there are kinds of chromosomes. In
only one case has a sufficient number of characters been studied to show
whether there is any correspondence between the number of hereditary groups
of characters and the number of chromosomes. In the fruit fly, Drosophila
ampelophila, we have found about 125 characters that are inherited in a
perfectly definite way. On the opposite page is a list of some of them.

It will be observed in this list that the characters are arranged in four
groups, Groups I, II, III and IV. Three of these groups are equally large
or nearly so; Group IV contains only two characters. The characters are put
into these groups because in heredity the members of each gro