SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 151 



previous investigators. A delicate line around the center fibrils was 

 believed to be a spirally wrapped filament or membranous sheath 

 enclosing the central pair of fibrils. 



Our own recent observations of amphibian and mammalian sperm 

 tails confirm the presence of pairs of "arms" on subfiber A and reveal 

 a significant difference in the density of the two subfibers of each 

 doublet (Fig. IB). A slightly greater density in the interior of sub- 

 fiber A was noted in the protozoan flagella studied by Gibbons and 

 Grimstone (1960) and a more noticeable difference has recently been 

 reported by Nagano (1960) in rooster sperm tails. The density of 

 subfiber A is quite marked in the sperm tails of rat, guinea pig (Figs. 

 5-7), and bat (Figs. 2 and 8). Indeed this subfiber may appear ho- 

 mogeneous in cross section whereas subfiber B appears tubular be- 

 cause it has a dense limiting layer around a center of low density. In 

 the salamander sperm, on the other hand (Fig. 3), the subfibers both 

 appear tubular, and a difference in density of their interior is scarcely 

 detectable. 



To date we have been unable to confirm the existence of a central 

 sheath or of a set of secondary filaments. Instead we find the central 

 pair of fibrils connected on either side by arcuate densities in the 

 matrix which do not seem to constitute an enveloping sheath (Fig. 

 IB). Other linear densities extend radially from these two curving 

 lines or from the central fibers themselves to subfiber A of the nine 



Fig. 2. An electron micrograph of a small area of the lumen of the 

 convoluted epididymal duct of a bat, Myotis lucifugus, showing, at the 

 right, a portion of the head of one sperm with the tip of its nucleus (Nc) 

 and acrosome (Ac); and, at the left, the tail of another spermatozoon cut 

 transversely through the principal piece. The axial filament complex is 

 surrounded by a thick fibrous sheath (Fs). Subfiber A (Sf-A) of each dou- 

 blet is distinctly denser than subfiber B (Sf-B). A faint radial pattern of 

 linear densities is discernible in the interfibrillar matrix. 



Fig. 3. Transverse sections of sperm tails of Tri turns viridescens. The 

 undulating membrane (Urn) arises as a broad, thin fold of the plasma 

 membrane attached at its base to the margins of a deep groove in the 

 axial rod (Axr) which is the core structure of the tail. The axial filament 

 complex (Afc) and marginal fiber (Mf) are found in the free edge of the 

 undulating membrane. The spokelike arrangement of linear densities 

 connecting the central pair to the peripheral doublets is especially clear 

 in this material. 



