Page:Popular Science Monthly Volume 83.djvu/190

186 cells which do not continue to divide throughout life, the size of cells varies directly with the body size and with the infrequency of division.

In a series of recent papers Richard Hertwig and several of his pupils have maintained that there is a definite ratio between the size of the nucleus and the size of the cell; this is the "Kernplasmarelation," or the nucleus-plasma ratio. When this ratio is altered by the greater growth of the nucleus, Hertwig thinks that it leads to a "tension," which brings about division, and thus the normal nucleus-plasma ratio is restored. This ratio is supposed to be a constant one under normal conditions, and if at any time it is altered it is capable of self regulation.

On the other hand, I have found that this ratio varies greatly in different cells of an animal, and indeed within the same cell at different stages of the division cycle, that it may be experimentally altered, and that it is a result, rather than a cause, of the frequency of cell division.

Within the same cell the size of the nucleus varies greatly at different stages of the division cycle, while the volume of the cell as a whole remains relatively constant. The nucleus is smallest during the anaphase, or later stages of division, when it consists of a compact plate of condensed chromosomes; it is largest immediately before the nuclear membrane dissolves at the prophase of the next division. In the cleavage of the egg of Crepidula plana the nucleus-plasma ratio in identically the same cell varies from approximately 1:6 whto the nucleus is largest, to 1:286 when it is smallest; that is, the volume of the nucleus increases nearly 50 times.during the resting period between the previous anaphase and the subsequent prophase; during this same time the volume of the cell remains practically unchanged.

Even when measured at the same phase of the division cycle the nucleus-plasma ratio differs greatly in different cleavage cells; at maximum nuclear size the volume of the nucleus of certain cells of Crepidula $$(4A - 4C)$$ may be 3 times that of the protoplasm, whereas in other cells $$(1A -1D)$$ the volume of the protoplasm may be 14.5 times that of the nucleus. At minimum nuclear size the nucleus-plasma ratio may vary from 1:29 in the cells $$1a^{2}-1d^{2}$$, to 1: 285.5 in the cells $$1A - 1D$$.

The growth of the nucleus between successive divisions is due to the absorption from the cell body of a particular kind of cell substance, which constitutes the achromatin of the nucleus; at the beginning of this growth the nucleus is composed of compact chromosomes, at its