CHAPTER I 



NUCLEUS AND CYTOPLASM 



In general a cell is composed of two principal morphological constituents, 

 the nucleus and the surrounding protoplasm, or better, cytoplasm, since 

 some authors use the former word to include both cytoplasm and nucleus. 

 This division of the cell constituents is quite sharp in all but the lowest 

 organisms, the nucleus being delimited from the cytoplasm by a membrane 

 except during a certain period of its division processes. In certain 

 unicellular organisms (Protista), however, the word " nucleus " is 

 inappropriate, since the material which composes this structure in the 

 higher organisms is scattered through the cytoplasm as minute granules, 

 or. chromidia (see Chapter VI.). In other Protista both nucleus proper 

 and chromidia are present {e.g. Difflugia), while certain Bacteria are said 

 to consist entirely of " nucleus." Even in multicellular organisms, both 

 in Metazoa and Metaphyta, there are frequently minute bodies in the 

 cytoplasm which are supposed by some cytologists to be derived from 

 the nucleus, and to consist of true nuclear material, and therefore to be 

 comparable to chromidia, 



A. THE CYTOPLASM 



The cytoplasm surrounding the nucleus includes a number of different 

 structures of which the following are the most important : 



(i) The cytoplasm proper. 



(2) The cell membrane. 



(3) The centrosome. 



(4) Chondriosomes. 



(5) Metaplastic bodies. 



(i) The Cytoplasm Proper 



The Hving cytoplasm itself consists of a viscid, nearly transparent 

 substance, which often is clearly not homogeneous. Great difference of 

 opinion exists as to its structure, chiefly owing to the fact that very little 



I B 



