Fig. 78.—Highly Magnified Tissue Cells from the Skin of a Salamander in an Active State of Growth. Dividing cells with chromosomes are shown at a, b, and c,. (After Wilson.)
Fig. 78 shows tissue cells, some of which are in the dividing and others in the resting-stage. The nuclei in process of division exhibit the rod-like chromosomes, as shown at a, b, and c.
Fig. 79.—Diagram of the Chief Steps in Cell-division. (After Parker as altered from Fleming.)
Centrosome.—The discovery (1876) of a minute spot of deeply staining protoplasm, usually just outside the nuclear membrane, is another illustration of the complex structure of the cell. Although the centrosome, as this spot is called, has been heralded as a dynamic agent, there is not complete agreement as to its purpose, but its presence makes it necessary to include it in the definition of a cell.
The Cell in Heredity.—The problems of inheritance, in so far as they can be elucidated by structural studies, have come to be recognized as problems of cellular life. But we cannot understand what is implied by this conclusion without referring to the behavior of the chromosomes during cell-division. This is a very complex process, and varies somewhat in different tissues. We can, however, with the help of Fig. 79, describe what takes place in a typical case. The nucleus does not divide directly, but the chromosomes congregate around the equator of a spindle (D) formed from the achromatin; they then undergo division lengthwise, and migrate to the poles (E, F, G), after which a partition wall is formed dividing the cell. This manner of division of the chromosomes secures an equable partition of the protoplasm. In the case of fertilized eggs, one-half of the chromosomes are derived from the sperm and one-half from the egg. Each cell thus contains hereditary substance derived from both maternal and paternal nuclei. This is briefly the basis for regarding inheritance as a phenomenon of cell-life.
Fig. 80.—Diagram of a Cell. (Modified after Wilson.)