54 



PROTOPLASM 



and much learned thereby, but such immersion experiments 

 are always limited by the selective and protective effect of the 

 protoplasmic membrane. If the salt is injected into the cell, 

 a different reaction may result from that taking place when 

 the cell is simply immersed in the solution. Injection is accom- 

 plished by micropipettes made just as are needles, except that 

 tubing is used instead of rod. A needle first results; the tip 

 is then broken off, leaving a tiny opening which, with practice, 

 may be made as small as 2 ju. The method of de Fonbrune 

 permits making the pipette directly and of any desired size. 

 The solution in the pipette is controlled by a plunger (syringe). 

 For the isolation of bacteria and spores, the mouth of the investi- 

 gator or a bulb gives sufficient control of pressure and suction, 

 but for microinjection, a more precise method is needed. A 



y///////////A 



Fig. 41. — The mechanically controlled micropipette of Taylor. 



hypodermic syringe, controlled by hand, is often used. Taylor 

 has devised a screw-controlled pipette which is capable of 

 great accuracy (Fig. 41). It is filled with mercury which is 

 forced outward or drawn inward by screws. The liquid to be 

 injected into a cell is first drawn into the tip of the pipette. 

 The pipette is then made to pierce the cell, and the liquid forced 

 into it. A little of the cell protoplasm may be withdrawn, or 

 a cell part such as the nucleus may be removed by applying suc- 

 tion to a pipette the tip of which is within a cell. 



A number of other methods for controlling micropipettes 

 have been devised, some involving the thermal control of the 

 expansion and contraction of mercury or air. The earlier 

 models of heat-controlled pipettes did not operate with precision, 

 but the newer one of de Fonbrune can be very accurately 

 controlled. 



Microelectrodes. — Attempts to measure the electrical poten- 

 tial of cells have led to many ingenious designs of microelectrodes. 



