26 



PRINCIPLES OF ANIMAL BIOLOGY 



The importance of the nucleus derives from a substance known as chro- 

 matin which it contains. This substance, as will appear in later chapters, 

 exercises some control over physiological processes, development, and 

 heredity. It owes its name to the fact that it colors deeply in most 

 ordinary dyes such as are used by cytologists to make it conspicuous 

 enough for study. The chromatin is collected into a number of distinct 

 masses, the chromosomes, but these bodies are so diffuse in their structure 

 that they cannot usually be recognized as separate objects except at the 

 time of cell division. During the periods between cell divisions one com- 

 mon form in which chromosomes exist is that of distended bags, the walls 

 of which contain the chromatin itself, while the interior is filled with a 



Vacuole 



Nudearsap 

 Chromatin 



Nucleolus- 



Nuclear 

 membrane 



Cell wall- 



Cell 

 membrane 



■Plasfid 

 Golgi body 



-4 Cenlriole 

 — Cenfrosphere 



Mihchondrla. 



Cell 

 inclusion 



Fig. 19. — Generalized cell. 



semiliquid substance called the nuclear sap. The chromatin is thus 

 greatly thinned out, though quite irregularly so, for there are little knots 

 and branching strands of it thick enough to be seen when stained. Some 

 chromosomes are shown in Fig. 38 (page 59), gradually experiencing this 

 expansion at the end of cell division. When the chromosomes in this 

 distended form are packed closely together in a nucleus, it is usually quite 

 impossil)le to see the outlines of the chromosomes, but the kn-ots and 

 strands of thicker chromatin are visible, together gi^^ing the appearance of 

 a network (Fig. 19). In other cells the chromosomes appear to be in 

 the form of branched threads rather than bags, but the resulting appear- 

 ance of the nucleus is still that of a chromatin network whose spaces are 

 filled with nuclear sap. 



Some nuclei contain, in addition to the sap and the network of 

 chromatin, a nucleolus. Two or more nucleoli may be present. They 

 are rounded bodies that stain readilv, but in a manner different from 



