14 THE ANIMAL CELL [CH. II. 



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 heat rigor. 

 This is due to the coagulation of the proteins present. 



b. 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 amosboid movements of the colourless 



corpuscles of the blood may be readily seen 



FIG. 15.-An ameboid cor- ^ 6n a dr P, f . bl d fr m the fin g 6r is mixed 



puscie of the newt killed with salt solution, and examined on a warm 



by instantaneous apph- , ji ,1 TO t i 



cation of steam, show- stage with the microscope. If a pseudopodium 



the pswdopodf a rai (Af ter f sucn a corpuscle is observed under a high 



schafer, " Quain's Ana- power, it will be seen to consist of hyaloplasm, 



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 



by a jet of steam allowed to play for a moment on the surface 



of the cover glass. Fig. 15 illustrates one fixed in this way. 



If, therefore, we adopt Schiifer's views on the structure of 

 protoplasm we see that the essential act in the protrusion of a 

 pseudopodium is the flowing of the hyaloplasm out of the spongio- 

 plasm; the retraction of the pseudopodium is a return of the 

 hyaloplasm to the spongioplasm. The spongioplasm has an irregular 

 arrangement with openings in all directions, so that the contractility 

 of undifferentiated cells may exhibit itself towards any point of the 

 compass. 



The relation of cells to various forms of stimulus has been recently very 

 extensively studied. Various forms of unicellular organisms have been used in 

 these experiments, and the stimuli employed have been chemical, thermal, light, 

 electric currents, and so forth. If the cell moves towards the source of attraction, 

 the term positive taxis is employed ; if it is repelled, negative taxis. The words, 

 chemo-taxis, thermo-taxis, photo-taxis, galvano-taxis, etc., indicate the kind of 

 stimulus! nvestigated. 



