148 DON W. FAWCETT 



and the tail wave is three dimensional. The nature of the sperm tail 

 movements thus continues to be a subject of dispute. 



The early electron microscopic studies of motile cell processes in 

 cross section revealed that the axial filament complex of cilia and of 

 sperm flagella is essentially identical, consisting of two central, single 

 filaments surrounded by nine evenly spaced, double filaments (Brad- 

 field, 1953; Fawcett, 1954). This finding posed the difficult problem 

 of accounting for the planar motion of cilia and the helical motion of 

 sperm tails with the same internal arrangement of filaments. In- 

 genious explanations were offered which assumed, in the case of 

 the cilium, a contraction of filaments on alternate sides to produce 

 pendular oscillations, and in the case of the sperm tail, a sequential 

 activation of the nine outer filaments initiating waves of shortening 

 that are propagated along the filaments out of phase so as to pro- 

 duce helical movements (Bradfield, 1955). For this explanation to 

 hold, it is necessary to assume the existence of two quite different 

 mechanisms at the base responsible for the two distinct patterns of 

 coordination in the contraction of the several core filaments. No 

 morphological basis for such a difference has been found. The prob- 

 lem of seeking a single satisfactory explanation for both ciliary and 

 sperm tail movements is greatly simplified if one adopts the view 

 that sperm tails, like cilia, execute bending movements that are 

 mainly in one plane. The fact that in sperm tails the bending move- 

 ments result in waves that are propagated from base to tip can, in 

 part, be regarded as a consequence of their considerably greater 

 length. In the discussion which follows, we shall proceed from the 

 assumption that sperm tail movements are mainly two dimensional, 

 although we realize there are some who will not share this view. 



THE AXIAL FILAMENT COMPLEX 



Since the description of the basic nine-plus-two fiber pattern of 

 cilia and flagella about eight years ago (Manton, 1952; Fawcett and 

 Porter, 1954), several additional details of the fine structure of the 

 fibrils and of the interfibrillar matrix have been discovered. Afzelius 

 (1959), working with sea urchin sperm tails, drew attention to a 

 slight inequality in the size of the two members of each doublet and 

 noted that the line through their centers was not on the circumfer- 



