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 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 chemistry.”  But why and how does this nuclear material determine sex?  In other words, what is the nature of the process of differentiation into male and female which it sets in motion?