A typical animal cell is commonly spherical in shape, but it may take a great variety of forms through compression. It has a cell-body or protoplasm, which is called also cytoplasm, especially when contrasted with the nuclear karyoplasm, and a nucleus. A few cells, like fat-cells and the human ovum, have an external covering membrane, or cell-wall. There is a part called the Centrosome observable in many cells, and this is made up of one or two minute dots surrounded by a radiating aster called the Attraction-Sphere. The centrosome is concerned in the process of cell-division and in the fertilization of the ovum; it is an important organ in the production of cell from cell, though its full nature and function are not yet known. The Plastid, or Protoplast, is another less important part found in certain cells; and in this by enlargement and differentiation are formed starch, pigment, and in some cases chlorophyl. Vacuoles are seen in cells; and there is an opinion that these may be a special kind of plastid: some vacuoles pulsate. The Nucleus is the most important part of a cell, the centre of its activity. The specific qualities of organism in origin and development are based upon nuclei, so far as the material element of the living cells is concerned. Vital stimuli pass through the nucleus into the surrounding protoplasm, and these stimuli control metabolism. The nutritive cytoplasm assimilates, but the vital principle energizes this assimilation through the nucleus, for a part of a cell deprived of the nucleus may live for a time, but it cannot repair itself. Constructive metabolism ceases when the nucleus is lost. A toxic disease like diphtheria kills by disintegrating cellular nuclei. 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. [Illustration: Fig. I A CELL. Throughout the Cytoplasm is a mesh containing numerous minute granules called Microsomes.]