THE ENERGY OF DIVISION The causes by which cell-division is incited and by which its cessation is determined are as yet scarcely comprehended, and the questions that they suggest merge into the larger problem of the general control of growth. All animals and plants have a limit of growth, which is, however, much more definite in some forms than in others, and which differs in different tissues. During the individual development the energy of cell-division is most intense in the early stages (cleavage) and diminishes more and more as the limit of growth is approached. When the limit is attained a more or less definite equilibrium is established, some of the cells ceasing to divide and perhaps losing this power altogether (nerve-cells), others dividing only under special conditions (connective tissue-cells, gland-cells, muscle-cells), while others continue to divide throughout life, and thus replace the worn-out cells of the same tissue (Malpighian layer of the epidermis, etc.). The limit of size at which this state of equilibrium is attained is an hereditary character, which in many cases shows an obvious relation to the environment, and has therefore probably been determined and is maintained by natural selection. From the cytological point of view the limit of body-size appears to be correlated with the total number of cells formed rather than with their individual size. This relation has been carefully studied by Conklin ('96) in the case of the gasteropod Crepidula, an animal which varies greatly in size in the mature condition, the dwarfs having in some cases not more than the volume of the giants. The eggs are, however, of the same size in all, and their number is proportional to the size of the adult. The same is true of the tissue-cells. Measurements of cells from the epidermis, the kidney, the liver, the alimentary epithelium, and other tissues show that they are on the whole as large in the dwarfs as in the giants. The body-size therefore depends on the total number of cells rather than on their size individually considered, and the same appears to be the case in plants.' Morgan has examined the same question experimentally through a comparison of normal larvae of echinoderms and Amphioxus with dwarf larvae of the same species developed from egg-fragments (95, ; '96). Broadly speaking, his results agree with 'Conklin's, though they show that the relation is by no means simple or constant. If unsegmented eggs of sea-urchins (Sphaerchinus) be shaken to pieces, fragments of all sizes are obtained which may segment and produce blastulas and gastrulas ranging down to the volume of the normal size. Dwarfs are also obtained from isolated blastomeres of two-, four-, or eight-cell stages. In both cases the number of cells in the blastula just before invagination is approximately proportional to the size of the blastula, though the smaller fragments show a tendency to produce a somewhat larger number, and their cells are, therefore, somewhat smaller than in the larger forms.
The same is true in Amphioxus. Morgan, therefore, draws the very interesting conclusion that " the ultimate size of the cells produced by repeated division determines when the division shall come to an end for a certain stage of the ontogeny." 1 This conclusion is, however, subject to exception ; for Morgan finds that the dwarf larvae show a tendency to use the same number of cells in the formation of certain organs as the full-sized individuals. Thus the dwarf blastulas tend to invaginate the same number of cells to form the archenteron as in the normal forms ; and in Amphioxus the notochord of the dwarfs consists in cross-section of three cells, as in normal individuals, irrespective of the total number of cells. It is clear, therefore, that there is another factor, besides the size of the cells, to be taken into account, and the whole subject awaits further investigation.
The gradual diminution of the energy of division during development by no means proceeds at a uniform pace in all of the cells, and, during the cleavage, the individual blastomeres are often found to exhibit entirely different rhythms of division, periods of active division being succeeded by long pauses, and sometimes by an entire cessation of division even at a very early period. In the echinoderms, for example, it is well established that division suddenly pauses, or changes its rhythm, just before the gastrulation (in Synapta at the 512-cell stage, according to Selenka), and the same is said to be the case in Amphioxus (Hatschek, Lwoff). In Nereis, one of the blastomeres on each side of the body in the forty-two-cell stage suddenly ceases to divide, migrates into the interior of the body, and is converted into a unicellular glandular In the same animal, the four lower cells (macromeres) of the eight-cell stage divide in nearly regular succession up to the thirty-eight-cell stage, when a long pause takes place, and when the divisions are resumed they are of a character totally different from those of the earlier period. The cells of the ciliated belt or prototroch in this and other annelids likewise cease to divide at a certain period, their number remaining fixed Again, the number of cells produced for the foundation of particular structures is often definitely fixed, even when their number is afterwards increased by division. In annelids and gasteropods, for example, the entire ectoblast arises from twelve micromeres segmented off in three successive quartets of micromeres from the blastomeres of the four-cell stage. In Echinus, according to Morgan, the number of cells used in the formation of the archenteron is approximately one hundred; in Sphcerechinus the number is approximately fifty.