3IO 



GENERAL PRINCIPLES 



cretions and other like substances resulting from the activity of 

 the protoplasm. With the last named class of cell constituents 

 may be included the cell membrane or cell wall. This is usually 

 present in plants and usually absent in animal tissue, though 

 many exceptions occur to both rules. 



673. The Nucleus. — The nucleus must be regarded as an 

 essential part of the cell. It is true there are certain lowly 

 organisms, such as the bacteria, blue green algae and related 

 forms, in which there is no such distinct, highly complex 

 structure as the typical nucleus; but even in these forms there 

 are found scattered in the protoplasm of the cell minute bodies 

 which have properties recognized as belonging to constituents 

 of the nucleus. These are generally regarded as representing 

 the nucleus. 



674. The typical nucleus is a round or oval body, but it may 

 also be greatly elongated or even branched. It is usually single, 

 but sometimes it consists of two parts, a large macronucleus 

 and a small micronucleus. Sometimes there are several or 

 even many nuclei in one cell. Usually the nucleus is provided 

 with a membrane, but this disappears at certain times, and in 

 some cases is entirely absent. Like the rest of the protoplasm, 

 the nucleus is transparent and colorless, and in the living condi- 

 tion appears homogeneous. But if the cell is treated with cer- 

 tain "fixing" and staining reagents, the nucleus becomes 

 deeply colored, due to the affinity of one of its constituents for 

 the dye. Because of its tendency to stain, this substance is 

 called chromatin. In the "resting" nucleus the chromatin 

 assumes a great variety of forms; sometimes it is in the form of 

 granules of various sizes, more often it is best described as a 

 mass of knotted and tangled threads. The chromatin is ap- 

 parently a very important constituent of the cell, and in it 

 centre many most interesting phenomena. Another element 

 of the nucleus which may be stained is the nucleolus. There 

 is often only one, but there may be more. They are usu- 



