They simply grow and divide in an ever alternating rhythm and this was the fundamental cadence in the song of life. If the large or mature stage is, in any sense, a prelude of old age, division in the same way represents rejuvenation. The latter is thus almost, although perhaps not quite, as primordial as the phase of growth itself, and among the most ancient and persistent of all the heritages that higher forms of life received from the lower is this power to grow young. Thus the systole and diastole of the heart of the _Zoölogos_ began. The monad becomes a duad; the individual, a dividual, almost as inevitably as the former grows; and the processional through this tiny life cycle contains in it the promise and potency of countless other processes that developed from it later. Thus even a colony of the far more complex coral polyps may develop perhaps for thousands of years from a single individual. Now, while protozoa may occasionally conjugate and thus prelude a higher form of reproduction and while the simpler metazoa may propagate by fission or budding, reminiscent of the older way, the general mode of propagation among many-celled organisms follows what seems at first a very different law. In these forms a sperm cell or spermatozoön must penetrate a germ cell or an ovum and then the zygote, or fertilized egg, immediately begins to reorganize itself from within and to divide into two, four, eight cells, etc.; and these divisions produce cells not all exactly like the mother cell but differentiation begins. In some species, as early as the first few divisions certain cells are set apart as germ cells, devoted exclusively to the purpose of reproduction. From these ova and spermatozoa arise. While others, far more in number and aggregate bulk and increasingly so as we ascend the scale of life, become more and more specialized for the production of different organs, structures, and tissues. These gradually lose the power to produce entire individuals. It is these that produce, and their descendants that constitute, all the rest of the body, or soma, and so are called the somatic cells. It is these and their progeny only that die while the germ cells, a very minute portion of the entire body in the higher forms of life, still continue, like the protozoa, to divide and grow _in sæcula sæculorum_, and it is they that, in a mundane sense, are immortal. Of course very few of the _circa_ four hundred ova produced during the sex life of an average human female and vastly less of the three hundred and forty billions of spermatozoa, according to Lote’s estimate, produced by the average male[167] become mature individuals. Most of them perish by the way and all those in the body at its death perish, of course, with it. But sex cells, or rather the germ plasm, even in the highest animals, including man, which attain their goal and produce mature individuals of a new generation, continuing to follow the old formula of eternal growth and division