268 A. E. HILTON ON THE CAUSE OF REVERSING CURRENTS 



(g) A very small plasmodium, or, which is the same thing 

 again, a very small piece cut from a larger plasmodium, is 

 capable of exhibiting streaming movements ; but the plasm 

 actually streaming is always less than the mass of which it 

 forms part. A certain relative proportion has evidently to be 

 maintained, because an excised portion exhibits streaming only 

 in places, although before excision its whole area may have 

 been in a streaming condition. 



All this clearly points to the conclusion that the streaming 

 of the interior plasm, liquefied by the damp substratum on which 

 the plasmodium moves, is controlled by drier aggregations of 

 plasm in contact with the atmosphere ; and that these con- 

 trolling centres affect the fluid plasm by an alternating force 

 of pressure and suction; the pressure propelling the streaming 

 plasm towards the extremities through the elastic extensions 

 of hyaloplasm, and the suction drawing it back again through 

 the same channels ; the hyaloplasm keeping the flowing plasm 

 intact, but at the same time yielding, in amoeboid fashion, to 

 its impulses. That pressure and suction are the cause of the 

 reversing currents I was able to verify by experiment. A 

 plasmodium was placed under a microscope in such a position 

 that the currents in the veins were easily observable, while 

 the plasm mass controlling the flow was sufficiently distant, 

 to the right of the objective, to allow a small and thin cover- 

 glass to be lightly laid upon it, without obstructing the field 

 of vision. Then, by bringing slight and momentary pressure 

 to bear on the controlling centre by gently tapping the cover- 

 glass with a needle, while watching the effects through the 

 microscope, I was able to superimpose upon the normal currents 

 additional movements exactly corresponding to the pressure 

 applied, and the rebound therefrom. Each tap on the cover- 

 glass caused the stream, according to the direction in which it 

 was flowing, to shoot or jerk a short distance backwards and 

 forwards, or forwards and backwards, before pursuing its normal 

 course. That the artificial pressure merely accentuated the 

 natural movements, without producing essentially unnatural 

 results, was attested by the fact that the delicate network 

 veins suffered no injury or alteration from the experiment. 



The question next arising is, of course, as to the origin of the 

 pressure and suction which cause the flowing movements. The 



