166 PROTOPLASM 



plasm is subjected not only to surface tension but to a uniformly 

 distributed tensile, or elastic, quality of the surface layer. Not 

 only will this property accomplish the same result as surface 

 tension in rounding up the incipient (fluid) cell wall, but it gives 

 a better mechanical basis for interpreting asymmetry, for it 

 owes its existence not to surface forces but to structural features. 

 If the droplet of protoplasm happens to be an amoeba, it does not 

 always round up into a ball any more than does the less active 

 yeast cell. Here surface forces are opposed. The Amoeba 

 assumes any shape that it "wants" to. It increases its surface 

 greatly. 



Fission, or the division of minute unicellular organisms such as 

 Protozoa, blue-green algae, and bacteria, may be the result of 

 surface activity in part. One can imagine a cell with a mem- 

 brane of relatively uniform tension throughout. Owing to 

 chemical changes within, the surface tension of the membrane 

 increases along a narrow band encircling the cell like a belt, 

 until the cell is pinched in two, just as would be the case if one 

 put a heavy rubber band around the imaginary equator of a toy 

 balloon. This may happen in simple cell division or fission, but 

 there is no conclusive proof of it. And thus it is with all our 

 surface-tension theories of vital processes. 



A Dublin physicist, Fitzgerald, was apparently the first to 

 suggest that muscle contraction is a surface-tension phenomenon. 

 Since then, others have advanced the same hypothesis, without, 

 however, being in agreement on whether the contraction involves 

 a decrease or an increase in surface tension. Fitzgerald argued 

 that if a mammalian muscle fiber is 0.000125 cm, (= 1.25 ^x) in 

 thickness, there would be about 500 m. of circumference of fibrils 

 per square centimeter of cross section of muscle. This, with a 

 surface tension equal to that of water, would give a disposable 

 force of 4 kg. as against the estimated value of 7 kg. for muscle. 

 By allowing for a surface tension less than that of water (Czapek's 

 estimation for protoplasm was a tension value two-thirds that of 

 water) and a smaller diameter of the fibrils, a much larger force 

 is made available. 



Bernstein argued along the same line and went to the trouble to 

 measure the size and estimate the number of muscle fibers, to 

 see if there was energy enough in their surface to account for 

 muscle contraction. He took the semimembranosus of a frog. 



