[Illustration: FIG. 7 A--Germ-cell (_p. g. c._) set apart in the eight-celled stage of cleavage in _Miastor americana_ (after Hegner). The walls of the remaining seven somatic cells have not yet formed though the resting or the dividing (_M p_) nuclei may be seen; _c R_, chromatin fragments cast off from the somatic cells. B--Section lengthwise of a later embryo of _Miastor_; the primordial egg-cells (_oög__{3}) are conspicuous (after Hegner).] =Germinal Continuity.--=The germ-cells are collectively termed the _germinal protoplasm_ and it is obvious that as long as any race continues to exist, although successive individuals die, some germinal protoplasm is handed on from generation to generation without interruption. This is known as the theory of _germinal continuity_. When the organism is ready to reproduce its kind the germ-cells awaken to activity, usually undergoing a period of multiplication to form more germ-cells before finally passing through a process of what is known at _maturation_, which makes them ready for fertilization. The maturation process proper, which consists typically of two rapidly succeeding divisions, is preceded by a marked growth in size of the individual cells. =Individuality of Chromosomes.--=Before we can understand fully the significance of the changes which go on during maturation we shall have to know more about the conditions which prevail among the chromosomes of cells. As already noted each kind of animal or plant has its own characteristic number and types of chromosomes when these appear for division by mitosis. In many organisms the chromosomes are so nearly of one size as to make it difficult or impossible to be sure of the identity of each individual chromosome, but on the other hand, there are some organisms known in which the chromosomes of a single nucleus are not of the same size and form (Fig. 8, p. 41). These latter cases enable us to determine some very significant facts. Where such differences of shape and proportion occur they are constant in each succeeding division so that similar chromosomes may be identified each time. Moreover, in all ordinary mitotic divisions where the conditions are accurately known, these chromosomes of different types are found to be present as pairs of similar elements; that is, there are two of each form or size.