SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 163 



termination of these two fibers the narrow edges of the columns pro- 

 ject into the spaces which they formerly occupied, and come into close 

 association with the corresponding doublets of the axial filament 

 complex (Figs. 6 and 7). 



It is only fair to say that this interpretation of the structure of the 

 fibrous sheath is not yet universally accepted. Several investigators 

 still deny the existence of any longitudinal components of the sheath 

 and insist that the thickenings observed in cross sections are confined 

 to the individual gyres of a helix (Schultz-Larsen, 1958; Anberg, 

 1959). This position seems to us difficult to defend in the face of the 

 evidence provided by longitudinal sections of sperm tails that pass 

 nearly tangential to the sheath (Figs. 15 and 16). In such sections the 

 successive circumferential elements of the sheath are seen to branch 

 and anastomose frequently in a manner quite inconsistent with the 

 interpretation of the sheath as a continuous helical wrapping. More- 



Fig 13. Two bat sperm tails sectioned transversely at different levels 

 in the principal piece. The circumferential elements of the fibrous sheath 

 (Fs) are wide in the first part of the principal piece (illustrated by the 

 section at the left), but become narrower toward the caudal end of this 

 segment (as shown in the cross section at the right of the figure). The 

 longitudinal columns of the sheath (Lc) are relatively inconspicuous in 

 this species and appear in cross sections as slightly denser areas on op- 

 posite sides of the sheath. 



Fig. 14. A longitudinal section through the first part of the principal 

 piece shows that the dense ribs of the sheath which appear broad in cross 

 sections of the tail are actually quite thin. Some are interrupted, others 

 branch (see at arrows) and connect with the next rib above or below. It is 

 evident that the older interpretation of this sheath as a continuous fiber 

 wound spirally around the longitudinal fibers of the tail was a great 

 simplification. 



Fig. 15. Two adjacent sperm tails in the bat epididymis. The section 

 has passed tangential to fibrous sheath of the uppermost sperm tail af- 

 fording a surface view of the dense circumferential ribs. These can be seen 

 to branch and anastomose (see arrows) in a manner that would not be ex- 

 pected of the successive gyres of a helix. The lower sperm tail in this figure 

 is cut in mid-saggital section, at a level near the end of the principal piece 

 where the ribs of the sheath are not so wide as they are in Fig. 14. 



Fig. 16. A section providing a surface view of the principal piece. This 

 clearly shows the ribs joining the slender longitudinal column of the 

 sheath (Lc). (From Fawcett, 1961.) 



