10 THE ANIMAL CELL. [CH. II. 



The Nucleus. 



In form the nucleus is generally round or oval, but it may 

 have in some cases an irregular shape, and in other cases there 

 may be more than one nucleus in a cell. 



The nucleus exercises a controlling influence over the nutrition 

 and subdivision of the cell ; any portion of a cell cut off from 

 the nucleus undergoes degenerative changes. 



A nucleus consists of four parts 



1. The nuclear membrane, which encloses it. 



2. A network of fibres in appearance like the spongioplasm 

 of the protoplasm but on a larger scale ; that is to say, 

 the threads of which it is composed are much coarser and 

 much more readily seen. The name chromoplasm has been 

 given to this network. 



3. The nuclear sap or matrix, a more fluid and homogeneous 

 substance which occupies the interstices of the spongework 

 of chromoplasm. 



4. Nucleoli ; these are of two principal varieties ; some are 

 knots or thickened portions of the network and others, the 

 true nucleoli, float freely in the nuclear sap. 



These four parts of the nucleus are represented in the next 

 diagram. 



Node of network _-,-_, Ifne . : ^ Nudear membiane . 



i Nucleolus. 



Node of network [rr^^P^i^TC-C^f- Nuclear matrix. 



\1 

 .W Nuclear network. 



Fig. 7. The resting nucleus diagrammatic. ("Waldeyer.) 



The next figure (fig. 8) gives a view of the nucleus, according to 

 the researches of Rabl. He considers that the fibres of the network 

 may be divided into thick fibres which he terms primary, and 

 thinner connecting branches which he terms secondary (shown 

 only on the right-hand side of the figure). This observer also 

 supposes that the primary fibres have the looped arrangement 

 depicted in the diagram. 



In the investigation of microscopic objects, a histologist is 

 nearly always obliged to use staining agents ; the extremely thin 

 objects he examines are so transparent that, without such stains, 

 much of the structure would be invisible. If such dyes as 



