Multicellular plants and animals do not reproduce by simple division, and the half of the parental body does not pass over into the progeny. Sexual reproduction is the chief means of multiplication in multicellular organisms, and in no case is it completely wanting; in most it is the only method of reproduction. In multicellular animals the power of reproduction is in the germ-cells, which differ from the somatic cells. Germ-cells do not maintain individual life as the body-cells do, but the germ-cells alone preserve the species. From two of these germ-cells under certain conditions is developed a complete bodily organism of the same species as the parents. These two cells are in a sense the undying cells; the somatic cells die.
Multicellular animals--Man, for example--grow embryologically by Mitosis or Indirect Division. As in Direct Division, typically, the nucleus in mitosis splits first and the cytoplasm secondly; but before the nucleus divides its content undergoes a series of changes. The chromatin loses its reticular arrangement and gives rise to a definite number of separate bodies, usually rod-shaped, known as Chromosomes. In this process the chromatin becomes a convoluted thread, called the Skein or Spireme. The thread thickens and opens out somewhat, and finally breaks transversely to form the chromosomes, which may be rods, straight, curved, ovoid, and sometimes annular. Commonly the nuclear material fades away and leaves the chromosomes in the cell-plasm. (Fig. II, 2 and 3.)
1. Cell with resting Nucleus. 2. Prophase: Chromatin in thickened convoluted threads, beginning of Spindle. 3. Prophase: Chromosomes. 4. Prophase: Spindle in long axis of the Nucleus, Chromosomes dividing. 5. Anaphase: Chromosomes moving toward the Centrosomes. 6. Chromosomes at the poles forming the Diaster, beginning splitting of the Cell-body. 7. Telophase, Daughter-Nuclei returning to resting state. 8. Daughter-Nuclei showing Monaster below. 9. The two new Cells.]
It is almost an established fact that each species of animal and plant has a fixed and characteristic number of chromosomes, which regularly recurs in the division of all its cells. In forms arising by sexual production the number is even. The number of chromosomes in the human cell is said to be forty-eight. There are, according to some observers, forty-seven chromosomes in man and forty-eight in woman. There seem to be twice as many chromosomes in white men as in negroes. Wilson gives the number[30] of specific chromosomes for seventy-four animals and plants. Germ-cells as differentiated from the somatic cells have in the perfected cell always half the number of chromosomes found in a somatic cell.