10 LIVING CELLS (2) 



Exercise 



systems; and to raise the question whether the 

 model and the organism behave similarly for 

 the same reasons. 



An artificial "ameba" 



Into a clean Syracuse watch glass laid on a 

 piece of white paper pour dilute nitric acid to a 

 depth of about ^ inch. Into this introduce a 

 drop of mercury about ^ inch in diameter; the 

 mercury is best introduced by putting the tip of 

 the pipet that contains it under the surface of 

 the nitric acid. Be careful not to spill any mer- 

 cury. Drop a crystal of potassium dichromate 

 about ^ inch in diameter or somewhat larger 

 into the nitric acid about f inch from the mer- 

 cury drop. You will immediately see the potas- 

 sium dichromate beginning to dissolve in the 

 nitric acid and diffusing from the crystal in all 

 directions. As the boundary of this yellow dif- 

 fusion zone reaches the mercury drop, things 

 begin to happen. Watch this for a time and 

 describe the phenomena you see. Are the mo- 

 tions you observe comparable in any way with 

 the mode of locomotion of an ameba? Have 

 you observed anything resembling cell division? 



The physico-chemical basis of this behavior is 

 as follows. Mercury has an exceedingly high 

 surface tension, the highest of any known liquid, 

 and for this reason assumes an approximately 

 spherical form on a surface, though flattened 

 by its own weight. The potassium dichromate 

 in nitric acid oxidizes the surface of the mer- 

 cury, lessening momentarily the surface tension 

 at this point, causing a local outflow of mercury. 

 Such points of oxidation, distributed asym- 

 metrically over the surface from moment to 

 moment, lead to the motions and cleavages you 

 have observed. 



(Go on watching this experiment as long as 

 you like. When you decide to clean up, be sure 

 not to drop any mercury on the floor or to let 

 any run into the sink. There will be a container 

 available into which to pour it. Mercury blocks 

 and rots plumbing; but much more serious is 

 mercury spilled around the room or on the 

 floors. There it enters the dust and may be 



inhaled or otherwise absorbed by the body in 

 this form. Since it is not readily excreted, the 

 body tends to accumulate it, and in larger 

 amounts it can produce very serious disturb- 

 ances. Make certain, therefore, that mercury is 

 not spilled, and that if any is spilled by accident, 

 it is immediately picked up. Any of it that is on 

 the floor can be brushed into a dust pan by 

 using a wet brush.) 



Ingestion, digestion, excretion 



In feeding, a protozoan exhibits some de- 

 gree of choice. Ordinarily a protozoan takes in 

 some objects and not others. Having taken in 

 a particle of potential food, the cell digests it in 

 part and excretes what remains. The process of 

 digestion is well understood, but the mechanisms 

 by which the organism ingests some objects and 

 excretes others are only partly understood. In 

 performing the following experiment we should 

 like you to note what analogies to these processes 

 it presents, and to ask yourselves to what degree 

 the simple mechanisms it involves are related to 

 the comparable phenomena in living cells. 



Put about 2 inches of distilled water into a 

 6-inch test tube, and drop into this 6 or 7 drops 

 of chloroform. Swirl the water in the test tube 

 and wait a minute for the chloroform to coalesce 

 at the bottom to form a single more-or-less 

 spherical drop about ^ inch across. Draw a 

 clean piece of glass rod, about ^ inch thick, to 

 a fine tip. Now attempt to insert the tip of the 

 rod into the drop of chloroform. Does the drop 

 accept it? Now wipe the rod dry, and dip the 

 very end into a solution of shellac. Blow on it 

 until it is dry. Now try again to make it enter 

 the drop of chloroform, watching closely what 

 happens. Does the drop accept it in the first 

 moment? later? 



Such a drop of chloroform under water, like 

 that of mercury, approximates a spherical shape. 

 It does so not so much because of its own sur- 

 face tension but because of the surface tension 

 of the water that surrounds it. Surface tension 

 is a force well described by its name: the mole- 

 cules of a fluid attract one another more or less 



