Before taking up the origin of the gametes from the spermatogonia and oogonia, it will be necessary to describe in some detail the complicated internal process involved in all typical cell divisions, known as Mitosis, which was dismissed when considering the origin of cells until the reader would be in a position to appreciate to the full its significance.
Reduced to its simplest terms, a typical resting cell, that is one which is not dividing, consists of a mass of cytoplasm surrounding a nucleus; the latter with its chromatin distributed so that it presents a net-like appearance. In addition to the nucleus, it will be recalled that there is present another important cell organ, the Centrosome, which appears like a tiny granule situated in the cytoplasm near the nucleus of the resting cell. For all practical purposes we may consider the cytoplasm as the arena in which mitosis takes place, the centrosome as the dynamic agent, and the nucleus, or more specifically its chromatin, as the essential element which the complicated process is particularly designed to distribute with nicety to the daughter cells which are in process of formation. With this in mind we may proceed to an outline of the chief stages of mitosis, first cautioning the reader to remember that variations in the details are as numerous as the different types of cells, and that any general account can do no more than present the fundamental plan of operations.
Stages of Mitosis
Broadly speaking, mitosis can be divided into four chief stages: Prophase, Metaphase, Anaphase, and Telophase, during each of which characteristic changes take place in the nucleus, cytoplasm, and centrosome.
At the beginning of the prophase, or earlier, the centrosome divides to form two, each of which becomes surrounded by what appears to be a halo (ASTER) of radiating fibers which are possibly cytoplasmic currents the visible expression of physicochemical forces.
The centrosomes and asters now proceed to move apart, take up positions at opposite sides of the nucleus, and the astral fibers between lengthen until they form a CENTRAL SPINDLE. While these changes are going on, the nucleus is not inactive. The nuclear membrane gradually disappears and the chromatin granules, originally in a net-like arrangement, seem to become rearranged in a more or less continuous thread of chromatin called the SPIREME. This, however, actually represents a number of definite chromatin entities, termed Chromosomes, which gradually by chromatin concentration become distinctly individual. The number of chromosomes varies greatly in different species, but is typically an even number and the same for all the cells of a given species.
When the chromosomes have assumed definitive form, the preliminary events which constitute the prophase of mitosis are brought to a close by the chromosomes being drawn to the center of the spindle. Here they are arranged in a plane at right angles to the long axis of the central spindle, midway between the two centrosomes, and form the EQUATORIAL PLATE.
And now the stage is set for what is apparently the climax of mitosis, designated the metaphase. Each of the chromosomes separates into two parts along the line of a longitudinal split, in such a manner that each of the thousands of chromatin granules which make up a chromosome is equally divided.
Two sets of similar daughter chromosomes are thus formed.
With chromosome division consummated, the metaphase merges into the anaphase which is devoted to a shifting of a daughter set of chromosomes along the fibers to either end of the spindle. In this way each centrosome becomes associated with one set of daughter chromosomes.
The last stage, or telophase, is one of nuclear reconstruction and division of the cytoplasm. The chromosomes be come indistinct as they spin out to form the net-like arrange ment of the chromatin in the nucleus of each daughter cell; a nuclear membrane arises; and the nucleus again assumes the form of a definite spherical body characteristic of the resting cell. It must be emphasized, however, that although the chromosomes usually disappear from view as definitive entities in the resting nucleus, nevertheless the individuality of each persists and the same chromosomes emerge from the nuclear complex at the next division period.
Simultaneously with these nuclear changes, and before the spindle and asters the machinery of mitosis disappear, the division of the cytoplasm is initiated as indicated by an indentation of the cell wall at the equator of the cell. This gradually extends through the cytoplasm in the same plane which the equatorial plate formerly occupied, until the cyto plasm is cut into two separate masses, each containing one of the daughter nuclei and centrosomes. And one cell has merged its individuality into two daughter cells by mitotic division.
A little thought will convince the reader that whereas the mitotic process apparently results in merely a mass division of the cytoplasm, the chromatin material is rearranged and distributed in a manner which makes it possible for each cell to receive a very definite share. Indeed this seems to be the primary object of mitosis. For in many cases there is a very great difference in the size of the resulting cells, but the number of chromosomes in each is the same. This, and other evidence which will presently appear, has clearly established the chromosomes as the chief factors in the transmission of characters from cell to cell, and therefore in inheritance.
