In the nucleus are several elements, the chief among which is Chromatin. Chromatin takes various forms, but commonly it is an irregular network. From the chromatin are derived the Chromosomes in the prophases of indirect cell-division which is the process of cell-division in the human body, except in lymph-cells and white blood-corpuscles, which split directly, or by Amitosis. Indirect cell-division is called Mitosis or Karyokinesis. In the male and female chromosomes, according to a common opinion of biologists, all the elements of parental and phyletic physical heredity are transmitted to the embryo.
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.