110 THE CELL 



instances of positive galvanotropism. It has been observed by 

 Verworn in Opalina ranarum, and in a few Bacteria and Flagellata 

 such as Cryptomonas and Ghilomcmas. When the circuit is com- 

 pleted the above-named species travel towards the anode instead 

 of towards the cathode, and collect there. If Ciliata and Flagel- 

 lata ate present side by side in one drop, they are seen under the 

 influence of the constant current to hasten in opposite directions, 

 so that finally two distinct groups are to be seen, the Flagellata 

 being at the anode, and the Ciliata at the cathode. If the current 

 is now reversed they advance like two hostile armies upon one 

 another, until they assemble again at the opposite poles. Each 

 time the current was made it produced in a few seconds a dis- 

 tinct sorting out of the crowd of Infusoria, which were otherwise 

 in inextricable confusion. 



IV. Mechanical Stimuli. Pressure, violent shaking, crushing, 

 all these act as stimuli to protoplasm. Weak mechanical stimula- 

 tions only produce an effect upon the point of contact ; strong stimuli 

 affect a larger area and produce a more rapid and more powerful 

 effect than weak ones. If a cell of a Tradescantia or Char a or the 

 Plasmodium of an zEthalium be violently shaken, or pressed npon 

 at one place, the granular movement is temporarily arrested, 

 whilst swellings and knots may even appear on the protoplasmic 

 threads, such as are produced by the electrical current. Hence it 

 frequently occurs, that in preparing the slide for observation all 

 the protoplasmic movements may be brought to a standstill, 

 simply by putting on the coverglass. They gradually return after 

 a period of rest. 



Amoebce and white blood corpuscles withdraw their pseudopodia 

 and assume a globular form when they are violently shaken. 

 Reticularia, which have extended their long processes, often with- 

 draw them with so much energy that the ends which were 

 attached to the slide are torn off (Verworn). A localised stimulus 

 can be produced at a given point with a fine needle. If the 

 stimulus is weak the effect is confined to this point, a varicosity 

 being formed and a shortening of the pseudopodium being pro- 

 duced. Strong and repeated stimuli cause neighbouring 

 pseudopodia, which were not directly touched, to contract (Fig. 

 57 B). 



If an Infusorian or other small animal comes in contact with 

 an outstretched pseudopodium, it is firmly grasped by it, and 

 becomes surrounded by the protoplasm. As the pseudopodium 



