Thus we can understand that the number of chromosomes remains the same in every cell-generation throughout development, as it is the same in all the individuals of a species. The numbers are known for many species: in some worms there are only two or four chromosomes, while in other related worms there are eight; in the grasshopper there are twelve, and in a marine worm, _Sagitta_, eighteen; in the mouse, the trout, and the lily there are twenty-four; in some snails thirty-two; in the sharks thirty-six, and in _Artemia_, a little salt-water crustacean, 168 chromosomes. In Man the chromosomes are so small that their normal number is not certain--sixteen have been counted. This counting can only be done during the process of nuclear division, for afterwards the chromosomes flow indistinguishably together, or rather apart, only to reappear, however, in the old form and number whenever the nucleus again begins to divide. It remains to be told what becomes of the centrosphere in cell-division. As soon as the formation of the daughter-nuclei has been brought about by the divergence of the split halves of the loops, the spindle figure begins to retrograde, its threads become pale and gradually disappear, as does the whole radiate halo of the centrosphere (Fig. _F_ and _G_). The cell-body has by this time also divided in the equatorial plane of the nuclear spindle, and the centrosome remains usually as a very inconspicuous pale body lying in the cytoplasm close to the nucleus, reawakening to renewed activity when cell-division is about to recommence (_G_, _csph_). These, briefly, are the remarkable processes of nuclear division. Their net result is obvious; the chromatin substance is divided between the daughter-nuclei with the greatest conceivable accuracy.