THE PHENOMENA OF LIFE. 



FIG. 3. Cell of Tradescantia drawn at 

 successive intervals of two minutes. The 

 cell -contents consist of a central mass con- 

 nected by many irregular processes to a 

 peripheral film : the whole forms a vacuo- 

 lated mass of protoplasm, which is continu- 

 ally changing its shape. (Schofield.j 



ing steadily in definite directions, some coursing round the film which 

 lines the interior of the cell-wall, and others flowing towards or away 

 from the irregular mass in the centre of the cell-cavity. Many of these 

 streams of protoplasm run together 

 into larger ones, and are lost in the 

 central mass, and thus ceaseless vari- 

 ations of form are produced. 



2. Irritability and the power of 

 response to stimuli. Although the 

 movements of the amoeba have been 

 described above as spontaneous, yet 

 they may be increased under the ac- 

 tion of various stimuli, and if the 

 movement have ceased for a time, as 

 is the case if. the temperature be low- 

 ered beyond a certain point, it may be 

 set up by raising the temperature. 



Again, contact with foreign bodies, gentle pressure, certain salts, and elec- 

 tricity, if applied to the amoeba, produce or increase the movement. It 

 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: 



1 % Changes of temperature. Moderate heat acts as a stimulant: this 

 is readily observed in the activity of the movements of a human colorless 

 blood-corpuscle when placed under conditions in which its normal tem- 

 perature and moisture are preserved. Extremes of heat and cold stop 

 the motions entirely. 



2. Mechanical stimuli. When gently squeezed between a cover and 

 object-glass under proper conditions, a colorless blood-corpuscle is stimu- 

 lated to active amoeboid movement. 



3. Nerve influence. By stimulation of the nerves of the frog's cornea, 

 contraction of certain of its branched cells has been produced. 



4. Chemical stimuli. Water generally stops amoeboid movement, and 

 by imbibition causes great swelling and finally bursting of the cells. In 

 some cases, however (myxomycetes), protoplasm can be almost entirely 

 dried up, and is yet capable of renewing its motion when again moist- 

 ened. Dilute salt-solution and many dilute acids and alkalies, stimu- 

 late the movements temporarily. 



Ciliary movement is suspended in an atmosphere of hydrogen or 

 carbonic acid, and resumed on the admission of air or oxygen. 



5. Electrical. Weak currents stimulate the movement, while strong 

 currents cause the corpuscles to assume a spherical form and become 

 motionless. 



3. Nutritive poiuers. The power of taking in food, modifying it, 

 building up tissue by assimilating it, and rejecting what i* not assimi- 

 lated. All these processes take place in the amoeba. They are effected 

 by its simply flowing round and inclosing within itself minute organisms 



