BIOMOLECULAR ASPECTS OF SPERMATOZOAN MOTILITY 227 



structural changes, and others (like the cross-/3 state when the mole- 

 cule is completely contracted) that are characteristic of the configura- 

 tional changes of the k-m-e-f group of proteins. 



The Position of the Flagellum in the Evolution of Movement 



The proteins present in the flagellum are distributed in some un- 

 known way so that their effect in producing wave motion cannot be 

 deduced. There is no evidence in the 9 plus 2 system for a sliding 

 mechanism of rigid elements (Gibbons and Grimstone, 1960); there 

 may not be any continuity of contractile protein at all, but rather at- 

 tachment of separate molecules in a geometric manner to some in- 

 soluble basement. In this respect, it must be pointed out that since 

 the flagellar protein may be highly solvated, shrinkage after fixation 

 and drying for examination in the electron microscope might tend to 

 create fiber-like artifacts from diffuse and swollen gels. This may be 

 true in muscles, and in flagella, certainly, considerable disagreement 

 has arisen already on the subject of the 9 thinner filaments reported by 

 Gibbons and Grimstone (1960) and others (Afzelius, 1959; Fawcett, p. 

 149). In one sense, the flagellum may represent more than a simplified 

 muscle. It may embody arrangements used earlier in the history of 

 movement, since there may have been an evolution of actin-like mole- 

 cules from simpler forms. Members of this molecular family may 

 have similar properties. They can be polymerized end-to-end and de- 

 polymerized to form an endless variety of structures. They can form 

 filaments alone, or they can be associated with more stable fibrous 

 structures (perhaps even in the complex mammalian sperm) acquir- 

 ing direction and purpose, particularly if enzymes or substrate-split- 

 ting sites are located at specific points in the fibrous (e.g., myosin) 

 partner. In muscle, the actin may be in the F-form permanently (Mar- 

 tinosi, Gouvea and Gergely, 1960); but elsewhere, the end-to-end 

 association might be temporary, allowing endless fabrications of 

 pseudopodia, reticulopodia, and other organelles as required. 



Originally, perhaps, these molecules may have been of the "porta- 

 ble" kind, but as they became more specialized for a particular type 

 of motion, the attendant biochemistry doubtless became more and 

 more complex and the reactive proteins were fixed relative to one 

 another. The flagellum may have been an early example of speciali- 

 zation directed toward rapid and controlled movement — the first 



