126 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



considerably in size and which are supported upon a linin framework 

 that does not stain by ordinary methods. The interstices of the 

 nucleus are filled with nuclear sap. A limiting nuclear membrane 

 may be present, but it is not an essential part of the nucleus. The 

 nuclear material may be all gathered together in a single mass, or it 

 may be distributed in small granules termed chromidia so that, at the 

 first glance, no nucleus seems to be present. Such chromidia may be 

 said to constitute a distributed nucleus, although the term nucleus is 

 usually applied to a well differentiated cell structure. 



The nucleus (page 15) is to be regarded as the most important unit 

 in the structure of the cell and is apparently essential for the con- 

 tinued existence of the latter. If cells are divided portions contain- 

 ing no nucleus invariably die while portions containing the nucleus 

 may continue to live and eventually recover from the injury. The 

 role of the nucleus is not fully understood but it seems certain that it 

 is a controlling center for the cell's activities. It is concerned in the 

 nutrition of the cell, frequently nuclear structures have to do with the 

 motility of cells and the chromatin serves as a medium for the 

 hereditary transmission of specific characteristics. Its functions, 

 therefore, are at least three-fold since it is active in trophic, kinetic 

 and reproductive capacities. Usually, all these functions are subserved 

 by a single nucleus; sometimes, however, as in the flagellates and 

 many ciliates they are divided between two nuclei (page 18). 



ACTIVITIES or THE PROTOZOA 



The higher animals or Metazoa are composed of a great number 

 of cells. A protozoon consists of a single cell. In the former the 

 various functions of the body are each carried out by a special type 

 of cell; for example, movement is performed by the muscle cells, 

 digestion is provided for by the cells of the alimentary tract, and urine 

 is excreted by the kidney cells. A protozoon being a unicellular 

 animal, these various functions must be performed within the single 

 cell of which it consists. Consequently certain parts of its protoplasm 

 are especially differentiated and function in a manner similar 

 to the organs of multicellular animals. Such differentiated parts are 

 termed organellce and by means of these the protozoa move about, 

 feed, and excrete waste products in many respects like the higher 

 animals. 



