34 " BIOLOGY 



larly through the nucleus; it may be in the form of stars, or a 

 long coiled thread, or it may appear as isolated threads, or as 

 threads interlaced, etc. Whatever its form, it always has the 

 power of absorbing coloring material and is probably always of 

 the same general, chemical composition. The nucleus controls 

 the cell activities, and the chromatin forms the most impor- 

 tant part of the nucleus. 



THE CENTROSOME 



Near the nucleus in many cells may be found a minute body 

 (Fig. 9cr) known as the centrosome (Gr. centron = center -f- 

 Gr. soma = body), which is usually present in the cells of 

 animals, where it seems to have an important function in con- 

 trolling the multiplication of the cell. The centrosome is 

 usually lacking in the cells of the higher plants. Frequently 

 two centrosomes are found near together, and sometimes they 

 are surrounded by a clear area, which is designated as the 

 centrosphere. At one time the centrosome was considered of 

 great importance in the life of the cell, from its prominent role 

 in cell division; but since it has been discovered that some cells 

 have none, while others have several, its significance as an 

 essential element in cellular structure has been doubted. 



THE CELL WALL 



One of the functions of the cell substance in many cells is to 

 secrete around the cell a material of harder consistency than the 

 protoplasm, the cell wall. Some cells have no cell wall; for 

 example, the animal shown in Figure 13 is a cell devoid of a cell 

 wall; and in many other animal cells the wall is either very 

 slight or entirely lacking. From this, it is evident that the cell 

 wall cannot be regarded as an essential part of the cell. In 

 nearly all vegetable tissues, the living protoplasm secretes a 

 membrane of greater or less consistency, and the same is also 

 true of many animal cells. The cell wall may be made of a 

 variety of different materials. In plants it is sometimes of wood, 



