178 LEONARD NELSON 



pendent of, but concerned with the control of, the contractile ele- 

 ments may be built into the flagellum. Tibbs' report (1960) that 

 fresh-water fish sperm heads contain acetylcholinesterase confirms 

 that of Sekine (1951), who stated that the sperm enzyme was nearly 

 as active as that of brain tissue. It may be fortuitous that the histo- 

 chemical incubation medium which contains a high concentration 

 of Na.,S0 4 to prevent diffusion of the cholinesterase appears to pre- 

 serve the axial core of the frozen-dried rat sperm flagellum which in 

 other media is quite labile. In fact, preliminary experiments which 

 I have been undertaking indicate that at least part of the axial core 

 may prove to contain cholinesterase activity. While this is at variance 

 with Tibbs' findings in trout and perch sperm, it is in agreement 

 with BradfielcVs prediction (1955) concerning the locus of a con- 

 ductile apparatus. A propos of the question of coordination and the 

 allied problem of guidance, Rothschild (1956) inquired several years 

 ago whether waves which travel along the tails can be bilaterally 

 asymmetrical when necessary. The consensus has been that periph- 

 eral fibers 1, 5, and 6 of mammalian sperm effectively determine the 

 plane of flagellation by virtue of their large size and their orientation 

 with respect to the plane of symmetry which includes the central 

 pair of longitudinal fibers (Fig. 5). Inasmuch as the other six fibers 

 (2, 3, 4, 7, 8, and 9) are also cytochemically positive for ATPase, in 

 spite of their less favored position vis-a-vis the "plane" of flagellation, 

 they could presumably act in the guidance of the sperm by contrib- 

 uting to the asymmetrical deformation of the flagellum (unless each 

 of the nine peripheral fibers is indeed sequentially involved in the 

 propagated undulations). 



Plasticity of the flagellum is undoubtedly a necessary condition 

 for wave generation. When ATP is depleted or washed out, flagella 

 frequently appear straightened or rigid in a manner akin to muscle 

 rigor. It is conceivable that in the undulating spermatozoon, the 

 passage of the undulatory wave temporarily and locally diminishes 

 ATP below a critical level so that a relaxation factor may be required 

 to intervene to maintain the plasticity during regeneration of the 

 ATP (Nelson, 1959a). The search for relaxing factors in muscle has 

 turned up a number of substances which behave as plasticizing agents, 

 including excess ATP, ATP-rephosphorylating systems (phosphoenol- 

 pyruvate or creatinephosphokinase), chelating agents, ATPase in- 



