PROTOPLASM -15 



Although nerve cells and epithelial cells show a relativelj^ high viscosity, 

 there appear to be less viscous channels within them. 



The viscosity of protoplasm may be experimentally decreased by 

 hydrostatic pressure or by stirring; it may be increased by anesthetics, 

 heat, electric currents, ultraviolet light, and X rays. These changes are 

 reversible, but extreme treatments may result in irreversible coagulation 

 or complete cytolysis. Experiments of this nature have demonstrated 

 the dependence of certain protoplasmic processes, notably cell cleavage 

 and the fascinating phenomenon of streaming with its various results, 

 upon orderly alterations in viscosity. One method of measuring ^-iscosity 

 is that of observing the rate of movement of nickel particles placed 

 within the protoplasm and then subjected to centrifugal or electromag- 

 netic force. 



Elasticit}', a characteristic of special importance 

 in view of the hints it affords regarding the ultrami- -»-=— 



croscopic structure of protoplasm, can be demonstrated 

 wdth the micromanipulator and by observing the tend- 

 ency of introduced nickel particles displaced by the 

 electromagnet to return to their original position when 

 the current is cut off. Red blood cells and certain 

 nuclei have been stretched between two micromanipu- 

 lator needles until they were, respectively, 4 and 25 j^-j^^ ;35— 8iir- 

 times their original diameter (Fig. 4), and upon release face layer of starfish 

 they returned to nearly their normal shape. Such ^th^ needle ^mT mi- 

 behavior may be due in part to the bounding mem- cro manipulator, 

 branes, but the nickel-particle technique shows clearly / er . m ers.) 

 that protoplasm itself, and not merely its membranes, is elastic as well 

 as ductile. 



Refined mechanical aids have made it possible to learn a great deal 

 about the phj^sical nature of the external membranes of cells. Echino- 

 derm eggs, because of their large size and other desirable characters, 

 have been used for this and many other types of cytological study. After 

 the removal of certain external coats, the protoplasmic surface film of 

 the egg lies exposed. The physical characteristics of this film can be 

 demonstrated by drawing it out in the form of a slender strand from the 

 egg surface and then allowing it to return to its original position (Fig. 35). 

 If not broken the material coalesces perfectly with the egg, while small 

 bits if broken away round up into droplets. This and a variety of other 

 treatments have shown that the surface membrane is elastic and water- 

 immiscible, though permeable; it shrinks without wrinkling, extends 

 without increase of surface tension, easily engulfs droplets of paraffin 

 oil, and undergoes rapid renewal fiom within when not too greatly 

 torn. 



