THE BODY ORGANIZATION 



and the nucleus actively participate, and that, as a result 

 of these changes, the contents of the old cell are rear- 

 ranged to form the new cells. Some of the different 

 stages in the process, as they have been studied under the 

 microscope, are indicated in Fig. 5. 



Gradually, through the formation of new cells and by 

 the growth of these cells after they have been formed, the 

 body attains its full size. When growth is complete, cell 

 reproduction is supposed to cease except where the tissues 

 are injured, as in the breaking of a bone, or where cells, 

 like those at the surface of the skin, are subject to wear. 

 Then new material continues to be added 

 to the protoplasm throughout life, but 

 in amount only sufficient to replace that 

 lost from the protoplasm as waste. 



Cell Surroundings. All cells are said 

 to be aquatic. This means simply that 

 they require water for carrying on their 

 various activities. The cells, in order 

 to live, must take in and give out 

 materials, and water is necessary to 

 both processes. It is also an essential 

 part of the protoplasm. Deprived of 

 water, cells become inactive and usually 

 die. Aquatic surroundings are provided 

 for the cells of the body through a liquid 

 known as the lymph, which is distributed 

 throughout the intercellular material (Fig. 6). This con- 

 sists of water containing oxygen and food substances in 

 solution. Besides supplying these to the cells, the lymph 

 also receives their wastes. Through the lymph the neces- 

 sary conditions for cell life are provided in the body. 



The General Work of Cells. In handling the materials 



FIG. 6. A tum- 

 bler partly filled with 

 marbles covered with 

 water, suggesting the 

 relations of the cells 

 to the lymph. 



