[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 group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.