REPRODUCTION.—We have noted above that the growth of an organism _which retains its shape_ alters the ratio of the surface area to the whole volume, so necessary for the changes involved in life. For the volume of an organism varies as the cube of any given diameter, whereas the surface varies with the square only. Without going into the arithmetical details, we may say that the ratio of surface to volume is lessened to roughly four-fifths of the original ratio when the cell doubles its bulk. As Herbert Spencer and others have pointed out, this must reduce the activities of the cell, and the due ratio is restored by the division of the cell into two.[33] This accounts for what we must look on as the most primitive mode of reproduction, as it is the simplest, and which we term "fission" at Spencer's "limit of {24}growth." Other modes of reproduction will be studied later (p. 30), after a more detailed inquiry into the structure of the nucleus and of its behaviour in cell-division. All cell-division is accompanied by increased waste, and is consequently _catabolic_ in character, though the _anabolic_ growth of living protoplasm, at the expense of the internal reserves, may be concurrent therewith. CELL-DIVISION In ordinary cases of fission of an isolated cell the cell elongates, and as it does so, like other viscid bodies, contracts in the middle, which becomes drawn out into a thread, and finally gives way. In some cases (_e.g._ that of the _Amoeba_, Fig. 4) the nucleus previously undergoes a similar division by simple constriction, which is called direct or "amitotic" division. But usually the division of the nucleus prior to cell-division is a more complex process, and involves the co-operation of the cytoplasm; and we must now study in detail the nucleus and its structure in "rest" and in fission.[34] We have noted above (p. 6, Fig. 2) the structure of the so-called "resting nucleus,"[35] when the cell is discharging the ordinary functions of its own life, with its wall, network of linin, chromatin-granules, and nucleole or nucleoles. The chromatin-granules are most abundant at two periods in the life of the cell, (1) when it is young and fresh from division, and (2) at the term of its life, when it is itself preparing for division. In the interim they are fewer, smaller, and stain less intensely. In many Protista the whole or greater part of the chromatin is densely aggregated into a central "nuclein-mass" or _karyosome_ {25}suspended in the linin network (long regarded as a mere nucleole). Such a nucleus is often termed a "vesicular nucleus".[36]