CH. II.] IRRITABILITY OF PROTOPLASM 15 



therefore called stimuli, and if the movement has ceased for the time, 

 as is the case if the temperature is lowered beyond a certain point, 

 movement may be set up by raising the temperature. Again, contact 

 with foreign bodies, gentle pressure, certain salts, and electricity, 

 produce or increase the movement in the amoeba. The protoplasm 

 is, therefore, sensitive or irritable to stimuli, and shows its irritability 

 by movement or contraction of its mass. 



The effects of some of these stimuli may be thus further 

 detailed : 



a. Changes of temperature. Moderate heat acts as a stimulant : 

 the movement stops when the temperature is lowered near the 

 freezing-point or raised above 40 C. (104 F.); between these two 

 points the movements increase in activity ; the optimum temperature 

 is about 37 to 38 C. Though cold stops the movement of proto- 

 plasm, exposure to a temperature even below C. does not prevent 

 its reappearance if the temperature is raised; on the other hand, 

 prolonged exposure to a temperature of 42-45 C. altogether kills the 

 protoplasm and causes it to enter into a condition of coagulation or 

 heat rigor. We have already seen that proteids, the most abundant 

 constituents of protoplasm, are coagulated by heat. 



I. Chemical stimuli. Distilled water first stimulates then stops 

 amoeboid movement, for by imbibition it causes great swelling and 

 finally bursting of the cells. In some cases, however (myxomycetes), 

 protoplasm can be almost entirely dried up, but remains capable of 

 renewing its movement when again moistened. Dilute salt solution 

 and very dilute alkalis stimulate the movements temporarily. Acids 

 or strong alkalis permanently stop the movements : ether, chloroform, 

 veratrine and quinine also stop it for a time. 



Movement is suspended in an atmosphere of hydrogen or carbonic 

 acid, and resumed on the admission of air or oxygen ; complete with- 

 drawal of oxygen will after a time kill protoplasm. 



c. Electrical. Weak currents stimulate the 

 movement, while strong currents cause the 

 cells to assume a spherical form and to become 

 motionless. 



The amoeboid movements of the colourless 

 corpuscles of the blood may be readily seen 

 when a drop of blood from the finger is mixed FIG. 

 with salt solution, and examined on a warm te 



stage with the microscope. If a pseudopodium 9 ati n of steam > show : 



, i i -i i 1 ' i ing the appearance of 



or such a corpuscle is observed under a high the pseudopodia. (After 

 power, it will be seen to consist of hyaloplasm, &% " Q " ai 

 which has flowed out of its spongy home, the 

 reticulum. Later, however, a portion of the reticular part of the 

 protoplasm may enter the pseudopodium. The cells may be fixed 



