THE STRUCTURAL BASIS OF THE BODY 35 



their immediate seat in the cytoplasm, the processes of constructive 

 metabolism which lead to the formation of new material, to the 

 chemical and morphological building up of the cell, are carried out in 

 or by the intermediation of the nucleus. 



HISTOLOGICAL DIFFERENTIATION OF CELLS. Even within 

 the limits of a single cell, differentiation of structure can take place by 

 the setting apart of distinct portions of the cell for isolated functions. 

 Thus in an organism such as vorticella the cell is shaped somewhat 

 like a wine-glass, the stem being composed of a spiral contractile fibre 

 which has the function of withdrawing the rest of the organism when 

 necessary towards its point of attachment. The main portion of the 

 cell presents at its free extremity a part which is the seat of ingestion 

 of food, and is therefore spoken of as the ' mouth.' This is surrounded 

 by a circle of cilia whose function it is to set up currents in the sur- 

 rounding fluid and so favour the passage of food particles towards the 

 mouth. Food when ingested at this end passes only a short distance 

 into the body of the vorticella. Here fluid is secreted around it which 

 serves for its digestion. This portion of the cell may therefore be 

 regarded as the alimentary canal or stomach. The indigestible residue 

 of the food is excreted in close proximity to the mouth. In addition 

 to these organs we have the usual differentiation of the protoplasm 

 into an external and internal layer, and the development within the 

 protoplasm of contractile vacuoles which serve to keep up a circulation 

 of fluid and therefore to pass the products of digestion through all 

 parts of the cell body. Within the limits of the single cell which forms 

 the vorticella we may therefore speak of organs for contraction, for 

 digestion, for circulation, and so on. 



The organs which are thus formed in unicellular animals or plants 

 can be divided into two classes, namely, (1) temporary organs, which 

 are formed out of a common structural basis and can therefore be 

 replaced at any time by the cytoplasm if destroyed. Examples of such 

 organs are the cilia, the commonest motor apparatus of unicellular 

 organisms ; the pseudopodia, which, as we have seen, can be made and 

 destroyed at will ; the mouth of animals such as Volvox or Vorticella ; 

 and the stinging cells or nectocysts, which surround the mouth of many 

 of these animals and serve to paralyse or kill the smaller living orga- 

 nisms w r hich are brought by the cilia within reach in order that 

 they may serve as food. In contradistinction to these organs are 

 (2) a number of others which must be regarded as permanent. These 

 cannot be formed by differentiation from the cytoplasm of the cell, 

 but are derived by the division of pre-existing organs of the same 

 character, and are therefore transmitted from one generation to 

 another. As examples of such cell organs may perhaps- be me-ntioned 

 the nucleus, with its chromosomes, and the plastids, of which the 



