SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 161 



sperm tails are additional contractile elements and not merely pas- 

 sive supporting structures. 



THE FIBROUS SHEATH OF THE PRINCIPAL PIECE 



The presence of a fibrous sheath around the principal piece of the 

 mammalian sperm tail was observed with the light microscope in the 

 latter part of the nineteenth century (Jensen, 1887). It was depicted 

 as a continuous fiber wound circumferentially around the axial fila- 

 ment. The early electron microscopic studies of intact sperm tails 

 shadowed with metal perpetuated the idea that the sheath consisted 

 of a single or double strand wound helically around the axial filament 

 complex, and the terms spiral sheath or tail helix were often used to 

 describe this component (Randall and Friedlaender, 1950). More re- 

 cent electron microscopic studies based upon observations on thin 

 sections have shown that this concept of the structure of the fibrous 

 sheath is greatly oversimplified and largely erroneous (Fawcett, 1958). 



In our opinion the sheath is not a continuous helical wrapping 

 but is made up of a series of semicircular ribs which insert into two 

 longitudinal columns that run along opposite sides of the tail (Figs. 

 5, 7, and 13). In cross sections of the principal piece of rodent sperm, 

 conspicuous thickenings are observed on opposite sides of the sheath 

 where the wider ends of the ribs merge with the longitudinal com- 

 ponents of the sheath (Figs. 5-7). The thickenings of the sheath are 

 always approximately in the plane of the central pair of fibrils, and 

 the tapering inner edges of the longitudinal columns project inward 

 toward fibers three and eiffht of the outer row. At levels distal to the 



Figs. 11 and 12. Two bat sperm sectioned longitudinally through the 

 base of the head, the neck, and anterior portion of the middle piece. The 

 dense, homogeneous heads have been sectioned parallel to their long 

 transverse diameter. The neck region contains a complex connecting 

 piece that appears to be a greatly modified centriole and shows a periodic 

 cross banded structure. Figure 11 is a median longitudinal section for 

 most of its length. Figure 12 is more obliquely cut, so that the section in 

 the lower half of the figure passes tangential to the mitochondrial sheath 

 revealing that the end-to-end junctions of the mitochondria in successive 

 turns of the sheath are aligned. Since these junctions were found in cross 

 sections to be in the plane of the central pair of fibers, one can infer that 

 that plane is perpendicular to the plane of flattening of the head. 



