166 DON w - FAWCETT 



It is a basic assumption of this approach that those components and 

 spatial relations which are of universal occurrence are essential to 

 motility, and those that are of more restricted occurrence are likely 

 to be merely variations related to the internal mechanics or the 

 peculiar locomotor problems of a particular species. The axial fila- 

 ment complex is characterized by a precise uniformity in the size 

 and position of its eleven fibrils, whereas in the outer coarse fibers 

 of the sperm tail, variation in length and diameter appears to be the 

 rule. This variation is not random however. Three large fibers, num- 

 bers 1, 5, and 6, are found in rat, guinea pig, cat, and man, and at 

 first it seemed likely that this pattern of size variation would prove 

 to be of general occurrence among mammals. Exceptions are now 

 beginning to appear. In the bat, for example, there are regularly 

 four of the dense outer fibers that are larger than the other five. These 

 are numbers 1, 9, 5, and 6 (Fig. 8). 



It is evident from examination of cross sections of sperm tails of 

 several species that although the basic organization of the fibrous 

 sheath is much the same, there are marked differences in its thick- 

 ness and in the cross-sectional profile of the tail (Fig. 17). The inter- 

 esting suggestion of Cleland and Rothschild (1959) based upon study 

 of the bandicoot (Fig. 17 A), to the effect that the distance of the outer 

 coarse fibers from the axial filament complex is related to the thick- 

 ness and stiffness of the sheath, is apparently borne out in the opos- 

 sum (Fig. 17B) where certain of the fibers of the outer row are placed 

 some distance away from the corresponding member of the inner 

 row. In rat and guinea pig (Fig. 17C and D) it might be argued that 

 the relatively greater diameter of fibers 1, 5, and 6 may overcome the 

 resistance of the sheath to bending, without moving the fibers farther 

 away from the hinge point. Examination of a cross section of the bat 

 sperm tail (Fig. 17E), however, leaves us in some doubt concerning 

 the validity of this line of reasoning. Here, despite an unusually 

 thick middle piece, and a very thick fibrous sheath in the principal 

 piece, the peripheral dense fibers are relatively slender, short, and 

 closely applied to the axial filament complex. It is clear that we still 

 have much to learn about the outer longitudinal fibers and the cir- 

 cumferential elements of the fibrous sheath and about their respec- 

 tive roles in sperm locomotion. Further comparative observations 

 may provide clues to their function but ultimately ways will have 



