CELLS AND THE CELL THEORY 



33 



olus (Fig. 9 no) is usually present and is sometimes very promi- 

 nent. The significance of this nucleolus is at the present day 

 unknown. 



The most remarkable substance in the nucleus is a material 

 known as chromatin (Gr. chroma = color); Fig. 9 ch. It has 

 received the name chromatin from the fact that it has a special 

 affinity for certain staining reagents, the chromatin material in 

 the nucleus being the first thing to absorb the color and become 

 stained. By special methods the chromatin may be stained and 

 the rest of the nucleus left unstained. The latter is sometimes 

 called achromatin (a = without + chroma = color). By this 

 special process of staining it is possible to show the chromatin 

 in prepared specimens, although in the living cell the chromatin 



FIG. 12. NUCLEI, SHOWING THE DIFFERENT APPEARANCES OF 



THE CHROMATIN (VARIOUS AUTHORS) 



is so transparent as to be practically invisible. Chromatin 

 occurs in a great variety of forms in different nuclei. Some of 

 these are shown in Figure 12. It is sometimes diffused irregu- 



