SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 155 



midpiece and principal piece of the tail are unusually thick in this 

 species. Electron micrographs of transverse sections show that the 

 nine coarse fibers do not vary appreciably in thickness and unlike 

 those of more flexible sperm, they are located a considerable distance 

 from the axial filament complex. Fibers 1, 5, and 6 in our system of 

 numbering are farthest from the axis of the tail, and 3 and 8 are 

 nearest. This outward displacement of the peripheral fibers was in- 

 terpreted as an adaptation to increase their effectiveness as tensile 

 elements by placing them farther from the hinge point (Cleland and 

 Rothschild, 1959). 



THE MITOCHONDRIAL SHEATH 



Among the points that remained controversial at the level of reso- 

 lution of the light microscope was the question as to whether the 



Fig. 4. Transverse section of a late spermatid of guinea pig in the 

 beginning of the midpiece. The outer coarse fibers are thickest in this 

 region and although they show some variation in shape, they are all 

 nearly the same size. 



Fig. 5. Transverse section of the principal piece showing that at this 

 level, only seven of the nine dense outer fibers remain. Numbers 3 and 8 

 have already terminated. There are now very marked differences in the 

 thickness of the fibers, with numbers 1, 5, and 6 much larger than the 

 others. The longitudinal columns of the fibrous sheath (Lc) are approxi- 

 mately in the plane of the central pair of fibrils. The difference in density 

 of the subfibers of each doublet is quite evident in this micrograph. 



Fig 6. A section of guinea pig testis transecting three sperm tails at 

 the level of the principal piece. The tapering inner edge of the longi- 

 tudinal columns of the fibrous sheath project into the spaces occupied at 

 a higher level by dense outer fibers 3 and 8, and thus divide the tail into 

 two unequal compartments, one containing three and the other, four 

 dense fibers. If the guinea pig sperm head is considered to be flattened 

 dorsoventrally the longitudinal elements of the fibrous sheath are be- 

 lieved to run along the dorsal and ventral aspect of the tail, and the tail 

 movements are believed to be mainly in the plane perpendicular to the 

 line through the center pair of fibrils in the axial filament complex. (From 

 Fawcett, 1961.) [Note added in proof. The direction of numbering of the 

 fibers should proceed from fiber 1 in the direction toward which the 

 "arms" on the doublets point. To follow this convention the position of 

 numbers 5 and 6 should be reversed. D. W. F.] 



