EPILOGUE 293 



The extent and nature of processes involving coordination within 

 spermatozoa have been rather neglected, owing largely, one suspects, 

 to the small size of the cells and the attendant technical difficulties. 

 This area of sperm investigation, however, would seem to warrant 

 extensive study. The commutator type of coordinating mechanism 

 for wave initiation, as postulated by Bradfield (1955) for example, 

 was probably erroneous, and there is little agreement on what might 

 account for wave propagation and coordination along the sperm 

 flagellum. Perhaps, as Brokaw notes for algal flagella, an extra co- 

 ordinating mechanism is not required; the sensitivity to elongation 

 of the contractile elements themselves might account for wave propa- 

 gation along the tail. It does seem, however, that in mammalian 

 sperm models, the capacity for coordination, along with wave propa- 

 gation, is lost completely, whereas contractility is retained (Bishop, 

 this symposium). 



The primary wave motion of normal fresh sperm is generally, if 

 not always, from the base of the flagellum toward the tip. Some 

 mechanism would appear to initiate this beat (whether it be two- or 

 three-dimensional), assuming that there is not a countermechanism to 

 inhibit the initiation of wave propagation elsewhere along the tail. 

 One might therefore expect to find periodic fluctuations of some kind 

 at the base of the head or in the midpiece, which periodicity would 

 correspond to flagellar rate: pulsed action potentials, cyclic meta- 

 bolic changes, fluctuations in ionic balance or other periodic varia- 

 tions, however rapid and however small. 



Just what role acetylcholine plays in sperm movement is quite ob- 

 scure at present, but the occurrence of this chemical transmitter has 

 been verified by both Tibbs and Nelson. Its unequivocal identifica- 

 tion and localization in sperm in general would suggest that a trans- 

 missible or coordinating mechanism is at work. 



The ultimate question — the nature of the mechanism of sperm 

 motility — remains at the end of this symposium, as at the beginning, 

 a major problem, and perhaps will continue so until another meeting 

 a decade hence. Much of the reasoning and most of the speculation 

 may be traced back to comparable processes and principles pertinent 

 to muscle contraction and implicate folding protein chains, sliding 

 filaments, oscillating fibrils, contractile enzymes, relaxing factors, 



