In 1903 and 1904 one of us, in conjunction with Professor Farmer, described the maiotic process in a variety of animals and plants. From the observations then accumulated it was in the first place shown that what we termed the maiotic process appears to be the same throughout the animal and vegetable kingdoms. In the second it was pointed out that the general scheme we were able to formulate was in accord with the particular description of the metamorphosis given by Korschelt for Ophryotrocha as long ago as 1895, as well as with the account of the same change in some amphibia given by Montgomery in the same year as ourselves. According to this conception of the maturation process, the reduction in the number of chromosomes to one half is brought about by a pairing of somatic chromosomes which takes place in the prophase of the first maiotic (heterotype) divisions. In this way we have in some mammals, for example, 16 pairs of chromosomes in the place of 32 single elements.