66 PROTOPLASM AND THE AMCEBA. PROTOCOCCUS 



and perhaps also carbon dioxide. Thus its function is 

 equivalent to that of the kidneys and bladder of the 

 higher animals, and partly perhaps to that of the 

 lungs. 



In the endoplasm is a rounded body called the 

 nucleus, which, as we shall see presently, is an all 

 important part of the protoplasm. The rest of the 

 protoplasm (ectoplasm and endoplasm together) is 

 called cytoplasm. Nucleus and cytoplasm together form 

 the structural unit of living organisation in the vast 

 majority of plants and animals. Such a unit is called 

 a cell, and the amoeba is commonly called a unicellular 

 animal because it consists of one such unit, in contrast 

 with multicellular animals and plants which consist of 

 many, in the largest forms of many millions. 



The nucleus is covered with a nuclear membrane (gel 

 membrane) and contains a substance chromatin, which 

 can usually be seen under the microscope in the form 

 of granules. The chromatin I is distinguished by its 

 power of taking up and becoming coloured by dyes 

 (stains), particularly certain stains (" nuclear stains "), 

 with great readiness. This probably takes place by 

 the adsorption of the particles of the dye by the dis- 

 perse phase of the gel of which the chromatin con- 

 sists. Chemically the chromatin is distinguished by the 

 presence in it of complex proteins containing phosphorus. 



The function of the nucleus in the life of the cell 

 can be inferred from its behaviour and from the be- 

 haviour of the cytoplasm in relation to it under various 

 conditions. Certain large Protozoa (unicellular animals) 

 can be cut into pieces under the microscope without 

 killing them. When this is done and one part of the 

 divided cell contains the nucleus while the others do 

 * Greek #pcu/*a, colour. 



