114 CELLS AND TISSUES 
when under the microscope, can be seen streaming around the cell 
wall or across the cell from side to side or end to end. 
The protoplasm consists of a number of structures which differ in 
organization, and each of which has one or more special functions. 
(Fig. 106.) One of the most conspicuous of these structures is the 
nucleus, which is a comparatively compact protoplasmic body, 
usually spherical in shape. Although usually centrally located 
in actively growing cells, the nucleus commonly has a lateral posi- 
tion in old cells. The nucleus is enclosed by a membrane, called 
the nuclear membrane, and is 
filed with a liquid known as 
nuclear sap, which consists of 
water and dissolved substances. 
However, nuclear sap is usually 
colorless and, therefore, not vis- 
ible. Within the nucleus also 
occur one or more small globu- 
lar bodies known as nucleoli 
(singular nucleolus) and much 
chunky or granular material 
known as chromatin, which is. 
regarded as the most impor- 
tant part of the nucleus and is 
Fic. 106.—A growing cell. w, cell ae named because it stains so 
wall; c, cytoplasm; », vacuoles filled readily when stains are applied 
with cell sap; 7, nucleus; a, nucleoli; to the cell. Around the nu- 
m, nuclear membrane; g, chromatin cleus and filling up the general 
granules. Enlarged about five hun- ¢avity within the cell wall is 
ae Ones that portion of the protoplasm, 
known as cytoplasm, which is a loose spongy structure full of many 
cavities called vacuoles. The vacuoles are filled with a liquid called 
cell sap, which like the nuclear sap consists of water containing 
dissolved sugars, salts, and other substances. The border of 
the cytoplasm is in contact with the cell wall and is modified 
into a membrane known as the cell membrane, which, since it is 
closely applied to the cell wall, can not ordinarily be seen until 
the cell is bathed in salt water or some other solution strong 
enough to shrink the protoplasm, so that the cell membrane is 
drawn away from the wall where it can be seen. (Fig. 107.) 
Within the cytoplasm commonly occur a number of small bodies 
