ELASTICITY 



233 



a taut rope suddenly severed. Finally, the stretched and now 

 contracted protoplasmic mass is slowly reincorporated into the 

 protoplast. This is a good indication that it is still living. Fur- 

 ther evidence that stretching does not necessarily cause severe 

 injury is to be had in active streaming. Protoplasmic streaming 

 often remains undisturbed in a cell after parts of it are stretched 



Fig. 112. — Protoplasmic strands formed by fibroblasts and other cells in tissue 



culture. 



to twenty times the original length or more. The elastic limit 

 of protoplasm may serve as a measure of the effects of salts 

 and other chemicals (page 448). 



That the elastic quality of protoplasm as demonstrated by 

 stretching with microneedles is by no means an abnormal one 

 is convincingly shown by the snapping and rapid contraction 

 of cell processes (pseudopodia, protoplasmic threads, and the 

 like) formed naturally by cells. 

 This is to be seen in tissue 

 cultures in which fibroblasts 



„ , J 1 • Flu. 113. — A leucocyte "pursuing" 



form long protoplasmic pro- ^^ abnormal red blood cell until almost 



trusions which adhere to the stretching itself in two. (Sketch from a 



, ,1 11 e 1 photograph by S. Mudd.) 



glass as the cell moves torward, 



then stretch and snap (Fig. 112). Observations of leucocytes by 

 S. Mudd show that the elasticity and tensile strength of cell 

 processes may be very great (Fig. 113). In all such cases, 

 the elastic quahty is indubitably a normal property, and it is 

 identical with that demonstrated by micromanipulative methods. 

 Entire cells, without cellulose walls, such as red blood cor- 

 puscles (Fig. 47), may be stretched between microneedles. 

 Where an entire cell is stretched, it is the elasticity of the cell 

 membrane that is determined; in fact, this is usually (but not 

 always) the case ; however, the plasma membrane is protoplasm. 



