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 _game_,
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
the new individual.

Neither the ovum nor the spermatozoon (the human race is referred to) is
capable alone of developing into a new individual. They must join in the
process known as fertilization. The sperm penetrates the egg (within the
body of the female) and the 24 chromosomes from each source, male and
female, are re-grouped in a new nucleus with 48 chromosomes--the full
number.

The chances are half and half that the new individual thus begun will be
of a given sex, for the following reason: There is a structural
difference, supposed to be fundamentally chemical, between the cells of
a female body and those of a male. The result is that the gametes (sperm
and eggs) they respectively produce in maturation are not exactly alike
as to chromosome composition. All the eggs contain what is known as the
"X" type of sex chromosome. But only half the male sperm have this
type--in the other half is found one of somewhat different type, known
as "Y." (This, again, is for the human species--in some animals the
mechanism and arrangement is somewhat different.) If a sperm and egg
both carrying the X-type of chromosome unite in fertilization, the
resulting embryo is a female. If an X unites with a Y, the result is a
male. Since each combination happens in about half the cases, the race
is about half male and half female.

Thus sex is inherited, like other characters, by the action of the
chromatin material of the cell nucleus. As Goldschmidt[1] remarks, this
theory of the visible mechanism of sex distribution "is to-day so far
proven that the demonstration stands on the level of an experimental
proof in physics or che