32 PHYSIOLOGY 



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 surrounding fluid and so favour the passage of food particles 

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



FIG. 12. Chromosomes of the germinal vesicle in the shark Pristiurua, at different periods, 



drawn to the same scale. (RiicKERT.) 



A. At the period of maximal size and minimal staining-capacity (egg 3 mm. in 

 diameter). B. Later period (egg 13 mm. in diameter). (7. At the close of ovarian life, 

 of minimal size and maximal staining-power. 



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 



