BIOMOLECULAR ASPECTS OF SPERMATOZOAN MOTILITY 219 



ficient cohesion between the filaments to prevent the otherwise solu- 

 ble protein from drifting apart during swelling. In this way the 

 individual behavior of the F-actin filaments was perhaps coordinated 

 to produce a network of reactive linear elements. 



DISCUSSION 



In considering the nature of contractility in living things, it is very 

 difficult to decide which factors are merely the embellishments of 

 evolution. In mammalian skeletal muscle there are so many biochemi- 

 cal and physiological features involved that it is scarcely surprising 

 that there are almost as many theories as facts. Even in simpler or- 

 ganisms, the electron microscope is now revealing such fine detail in 

 the structure of contractile organelles that it seems likely, from a mo- 

 lecular standpoint at least, that we may never find a "primitive" 

 living contractile system, particularly since locomotion is a major 

 factor in the struggle for existence and ancient forms of movement 

 may have been swallowed up long ago by their more efficient descend- 

 ants. In this scheme of things, however, the flagellum seems to have 

 remained, by tacit phylogenetic agreement, as a blueprint satisfactory 

 enough to stand the test of time. There is no reason to suppose that 

 unicellular animals swam in primeval seas with any different 9 plus 

 2 flagella than they do today, and the modern flagellum, however 

 modified, might still contain those common denominators once used 

 to improve motility and to develop more advanced muscles. 



Models of Contraction 



Very broadly, two models of contraction have been proposed as a 

 basis of biological movement — those based on the relative movements 

 of rigid elements and those based on the folding or bending of some, 

 or all, of the contractile substances. These models depend on differ- 

 ent data, some morphological, some physical and physicochemical, 

 and some on thermodynamic argument. There is sufficient evidence, 

 however, to indicate that most of the living contractile systems that 

 have been studied have a common link somewhere, and if this is so, 

 any really fundamental explanation of contraction must apply equally 

 to a wide range of motile organs and organelles; and moreover, the 

 explanation should also fit contractile systems that have been re- 

 constituted from extracted components, often from different animals. 



