38 PRINCIPLES OF ANIMAL BIOLOGY 



like band in spiral form, as in some cells of insects; of a coarse rope-like 

 strand surrounded by an occasional ring, as in the cells of the salivary 

 glands of midges; or it may be so compact as to conceal all appearance 

 of a network, as in many of the Protozoa. 



The chromatin is the most important constituent of the nucleus, 

 and indeed of the whole cell. It determines to a large extent, as is shown 

 in subsequent chapters, the nature and activities of the cell. It is not a 

 uniform material, but is differentiated into a number of chemically differ- 

 ent substances. These different components are arranged in definite 

 order in the chromomeres distributed along the hnin network, and 

 mechanisms operate in the division of cells and in the origin of repro- 

 ductive cells to maintain this order, or to change it only in limited 

 fashion. 



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

 and chromatin, a nucleolus {nl). Two or more nucleoli may be present. 

 They are rounded bodies that stain readily, but in a manner different 

 from the chromatin. Nucleoli are therefore not to be confused with net- 

 knots, which are sometimes unwisely called nucleoli. The nature and 

 function of the nucleolus, when it is present, are not understood. Some 

 biologists have regarded it as a waste product; others have held it to be a 

 reserve supply of materials used in cell division, since it disappears during 

 that process; and recently it has been regarded as a reserve food supply 

 for the nucleus. 



Chemistry and Physics of Protoplasm. — Not less important than the 

 structural features of cells is the chemical composition of their proto- 

 plasm. This composition is so complex, however, that it must be de- 

 scribed in rather general terms. It appears that the following elements 

 are always present in every cell: carbon, hydrogen, nitrogen, oxygen, 

 phosphorus, sulphur, sodium, potassium, magnesium, calcium, iron, and 

 chlorine. In addition to these, aluminium, silicon, manganese, copper, 

 fluorine, bromine, and iodine are sometimes present. These lists apply 

 to the cell as a whole. When different portions of the cells are considered 

 by themselves, it is possible to reduce the number of elements somewhat. 

 Thus, it is commonly stated that the only elements always found in 

 protoplasm (not cell) are carbon, hydrogen, oxygen and nitrogen; but 

 that often phosphorus and sulphur are likewise present. Such state- 

 ments, however, probably ignore the salts dissolved in the liquids of the 

 protoplasm. It is unlikely that these salts are ever whollj^ lacking. 

 However, since it is not the presence of certain elements, but the ways in 

 which they are combined, that make protoplasm what it is, it is not impor- 

 tant to agree on either the minimum or maximum number of elements 

 that enter into its composition. 



Proteins. — The principal constituents of protoplasm are substances 

 of the class called proteins. These substances are abundant in lean 



