The =cell wall= which surrounds the protoplast is a product of its activity. The structure and composition of the wall of any given cell vary according to the ultimate function of the cell. The walls may be thin or thick, porous or non-porous, and colored or colorless. The composition of cell walls varies greatly. The majority of cell walls are composed of cellulose, in other cells of linin, in others of cutin, and in still others of suberin, etc. In the majority of cells the walls are laid down in a series of layers one over the other by apposition, similar to the manner of building a pile of paper from separate sheets. The first layer is deposited over the primary wall, formed during cell division; to this is added another layer, etc. A modification of this manner of growth is that in which the layers are built up one over the other, but the building is gradually done by the deposit of minute particles of cell-wall substance over the older deposits. Such walls are never striated, as is likely to be the case in cell walls formed by the first method. In other cells the walls are increased in thickness by the deposition of new wall material in the older membrane. The cell walls will be discussed more fully when the different tissues are studied in detail. INDIRECT CELL DIVISION (KARYOKINESIS) The purpose of cell division is to increase the number of cells of a tissue, an organ, an organism, or to increase the number of organisms, etc. Such cell divisions involve, first, an equal division of the protoplast and, secondly, the formation of a wall between the divided protoplasts. The first changes in structure of a cell undergoing division occur in the nucleus. CHANGES IN A CELL UNDERGOING DIVISION The =linin threads= become thicker and shorter. The =chromatin granules= increase in size and amount; the threads and chromatin granules separate into a definite number of segments or =chromosomes= (Plate 1, Fig. 2). The nuclear membrane becomes invested with a fibrous protoplasmic layer which later separates and passes into either end of the cell, there forming the =polar caps= (Plate 1, Fig. 3). The =nuclear membrane= and the =nucleoli= disappear at about this time. Two fibres, one from each polar cap, become attached to opposite sides of the individual chromosomes. Other fibres from the two polar caps unite to form the =spindle fibres=, which thus extend from pole to pole. All these spindle fibres form the =nuclear spindle= (Plate 1, Fig. 5).