THE CELL 25 



In some unicellular organisms movements take place along 

 some definite line, and fibrils are found arranged more or less 

 parallel to the line of movement. Such contractile processes, 

 from their resemblance to muscles, have been termed myoids. 

 In other protozoa the pseudopodia manifest a to-and-tro 

 rhythmic waving movement, which may cause the cell to be 

 moved along, or may cause the adjacent fluid to move over 

 the cell. Such mobile processes, when permanent, have been 

 called cilia. These movements are the result of chemical 

 changes in the protoplasm, by which alterations in the 

 osmotic pressure and changes in the surface tension of the 

 various parts are produced. 



The movements are modified by the various Stimuli 

 which alter the activity of the chemical changes (p. 10). 

 The stimulation may be (a) General. — Cooliog diminishes 

 and finally stops them. Gentle heat increases them, but 

 when a certain temperature is reached they are stopped. 

 Drying and various drugs, such as chloroform, also arrest 

 the movements. 



(6) Unilateral. — Changes in the surroundings may cause 

 either contraction or expansion, may repel or attract. When 

 an attracting or a repelling influence, a positive or a negative 

 stimulus, acts at one side of the cell — unilateral stimulation — 

 it may lead to movement of the cell away from it or towards 

 it. If the action is towards the stimulus, it is said to be 

 positive ; if away from it, negative. 



Chemiotaxis is the attraction or repulsion produced by 

 one-sided application of chemical stimuli. This is well seen 

 in the plasmodial masses of cethalium septicum, which grows 

 on tan. Oxygen and water both attract it, exercising a 

 positive chemiotaxis. It is also seen in the streaming of 

 the white cells of the blood to disintegrating tissues, or to 

 various micro-oreranisms introduced into the tissues which 

 have to be destroyed to prevent their poisoning the organism, 

 and in the attraction exercised by the ovum upon the male 

 element in reprodiiction. 



Barotaxis is the effect of unilateral pressure or mechanical 

 stimulation. Many protozoa appear quite unable to leave 

 the solid substance — e.g. the microscope slide — with which 



