Fig. I
A Cell.
Throughout the Cytoplasm is a mesh containing numerous minute granules called Microsomes.
The production of cell from cell is accomplished either by direct splitting of the nucleus and cytoplasm into two new cells, or by indirect division through a series of stages. In a typical direct, or amitotic, division the nucleus is constricted in the middle and divides into two daughter-nuclei. These by amoeboid movements withdraw to the poles of the cell; the cell finally divides between them, and thus two cells are formed. These, again, split into four, the four into eight, and so on. An amoeba by direct division can separate into two distinct new animals in ten minutes.
Heredity here is simple. In unicellular organisms, such as Rhizopoda and Infusoria, each individual grows to a certain stage, and then divides into two parts, which are exactly alike in size and structure, so that it is not possible to decide whether one is older or younger than the other. These organisms reduce the size of their overgrown bodies by division. Each individual of any such unicellular species is a part split off serially from an organism which started into life ages ago. Some of them have come down in uninterrupted life from geological epochs that passed away eons before the first man was created. Many of these unicellular plants and animals have immeasurably the most ancient form of life on earth. Heredity with them depends upon the fact that each offspring is merely half of its parent. In some cases the division has a sexual quality: two cells in Paramecium, and, like Infusoria fuse and then divide if they come into contact; they can, however, split without this sexual process.
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.)
