EPILOGUE 291 



of flagellation is not likely to endear us to those who still believe that 

 sperm have to swim the distance from emitter to egg, but there ac- 

 tually is little direct evidence that active motility is required by 

 sperm of most animals at times other than the precise moment of 

 fertilization. While most of us do not envisage spermatozoa as free- 

 ranging, voracious predators, the selection by Carlson of food collec 

 tion as the limiting parameter to energy expenditure seems reasonable. 

 Exogenous substrate, and even molecular oxygen, within the genital 

 tract of either sex can conceivably function as an essential prerequi- 

 site for sperm survival and could, under conditions of short supply 

 or limited diffusion, constitute the controlling factor in establishing 

 the balance between energy expenditure by movement and energy 

 conservation through quiescence. With respect to spermatozoa, Carl- 

 son's postulates would involve survival rather than growth, and the 

 selective value of any mechanism concerning energy balance would 

 not be inconsistent with the genetic and physiological variability 

 which occurs within sperm populations (see Bishop, 1961). 



The contributions of Nelson, Pautard, Tibbs, Brokaw, and Bishop 

 have brought up to date the status of the contractile proteins which 

 can be extracted from, or demonstrated in, fiagella. Depending on 

 the source material, method of extraction, conditions of assay, and 

 biochemical properties, these products appear roughly comparable 

 to actin, myosin, or actomyosin. Their physical characteristics where 

 known, response to ATP, splitting activity, ionic requirements, inter- 

 action with muscle proteins, and sensitivity to inhibitors and chelat- 

 ing agents suggest close parallels with functional proteins from mus- 

 cle. The final characterization and identification, however, of none 

 of these components are as yet complete, and forthcoming investiga- 

 tions must decide how distinctive or how similar to muscle proteins 

 these sperm constituents may be. With only rare exception is the 

 precise site of contractile material in the flagellum indicated — that 

 localized by means of the antibody-absorption technique, com- 

 bined with electron micrography, as demonstrated by Nelson. Gen- 

 erally, little change occurs in fibrillar ultrastructure with extraction 

 of contractile components. 



A very significant finding is Pautard's claim for an oscillating gel, 

 extractable from fish sperm by a modified Weber-Edsall procedure. 

 Whether this proves to be a new type of "oscillating protein," char- 



