Tissues are grouped together to form organs. Every cell consists of a cell body and a nucleus. The cell body consists of a substance known as protoplasm. The nucleus is the essential part of a typical cell and is the controlling center of its activity. [Illustration: FIG. 1—A, A vertical section of the cuticle; B, the lateral view of the cells; C, the flat side of scales like (d) magnified 250 diameters.] Cells divide or reproduce themselves by means of direct or indirect division. In direct division the nucleus and the cell wall simply divide into two equal divisions and results in the formation of two new cells. In indirect division the process is much more complicated, and several stages must be passed through before there is a complete division. The process of fertilization consists in the conjugation of two sexual cells. The male sexual cell is called the spermatazoon, and the female sexual cell is called the ovum. The nucleus of the ovum in its earlier development stages is known as the germinal vessicle. In the living organism many cells are destroyed during the various physiologic processes and are replaced by new ones. When a cell dies, changes take place in the nucleus which result in its gradual disappearance. This process is known as chromatolysis. =Tissues.=—A _tissue_ is an aggregate of cells all having a common function. Those important tissues with which the embalmer should be more or less acquainted are the following: Skin, nails, hair, superficial fascia, deep fascia, lymphatics, glands, cartilage, bone, teeth, nerves, muscles, tendons, aponeuroses, ligaments, fat, mucous membranes, serous membranes, synovial membranes, arteries, veins and blood. =The Skin.=—The skin or integument (intego, to cover) is the outside covering of the human body. It is the first tissue that is cut when operating upon the body. [Illustration: FIG. 2—A cross section of the skin. (Gray)] The skin is the seat of the organs of touch. The multitudes of sensory nerve endings convey the sensations of temperature, pressure and pain to the brain, thus informing the brain at all times, to keep the body from harm, and in a strong and healthful condition. The skin is also the regulator of the body temperature, for connected with the skin are sweat glands, and sebaceous glands, each having important excretory functions. The skin is also a protective coat, very elastic, and varies greatly in thickness. It is thinnest in the eyelids and thickest over the back of the neck, back of the shoulders, palms of the hands and the soles of the feet.