Page:Popular Science Monthly Volume 83.djvu/192

188 ; up to a certain stage, the nuclei of such cells shrink in size and become more densely chromatic in proportion as the cell protoplasm is converted into metaplasm. The same thing is true of gland cells, muscle cells, fiber cells and fat cells in which the general protoplasm is progressively being changed into differentiation products, and coincidentlycoincidentally [sic] the individual nuclei shrink in size and become more densely chromatic.

In no case do metaplasmic substances or differentiated structures of the cell enter into the nucleus during its growth, and the relative quantities of general protoplasm and of differentiated products in a cell can be determined by the size to which the nucleus will grow during interkinesis, under given conditions of time, temperature, etc. By subjecting eggs to centrifugal force, the quantities of protoplasm and yolk in the cleavage cells may be greatly changed, and under such circumstances the size of a nucleus is always proportional to the volume of the protoplasm in which it lies; the heavier yolk which segregates at the peripheral pole, and the lighter watery or oily substance which gathers at the central pole of the centrifuged egg do not contribute to nuclear growth, only the clear protoplasm which lies in the middle zone enters the nucleus or contributes to its growth. In muscle cells with small nuclei, the quantity of general protoplasm (sarcoplasm) which may enter into the nucleus or contribute to its growth is small; in nerve cells, it is evidently larger, since the nuclei of such cells are relatively large, but the substance which may enter the nucleus of a nerve cell is by no means as great in quantity as in germ cells and blastomeres, thus indicating that much of the substance of a nerve cell is too highly differentiated to enter into the nucleus. In epithelial and gland cells, the size of nuclei is limited not only by the presence of metabolic products in the cells, but also by the occurrence of cell division and the consequent limitation of the growing period of the nucleus.

The following table gives the cell diameter and nuclear diameter at maximum size, the corresponding nuclear volume, the cell volume less the nuclear volume, and the nucleus-cell ratio, in a number of different kinds of cells in adult individuals of Crepidula plana:

The nucleus-cell ratio of these cells varies from 1:1.3 to 1:88.6, depending primarily upon the quantity of formed substance in the cells. The nuclei arc relatively largest in germ cells before the formation of yolk, and in embryonic cells in which there is relatively little formed substance; in such cases a relatively great part of the protoplasm may enter the nucleus. The nuclei are relatively smallest in those cells in which the protoplasm has been most completely transformed into products of metabolism or differentiation, such as gland cells filled with secretion, red blood cells of mammals in which the nuclei completely disappear, egg cells filled with yolk, and spermatozoa in which most of