SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 157 



mitochondria of the sperm middle piece coalesce late in spermiogen- 

 esis into a continuous strand wrapped spirally around the base of 

 the flagellum, or whether they retain their individuality. It is now 

 clear from the study of electron micrographs that either may occur. 

 In insects, gastropods, and other invertebrates the conspicuous ne- 

 benkern formed during spermiogenesis arises by fusion of mito- 

 chondria. It subsequently spins out into a long mitochondrial strand 

 that becomes intimately applied to the axial filament complex of the 

 tail. In the vertebrates, and particularly in mammals, the mitochon- 

 dria generally remain separate but there are marked species differ- 

 ences in their form and arrangement. In some species they are spheri- 

 cal, in others they are elongated and arranged end to end in a heli- 

 cal sheath around the fibrous core of the tail (Fig. 9). The propor- 

 tions of the midpiece, the length of the individual mitochondria, and 

 the pitch of the helix vary greatly from one mammalian species to 

 another. The mitochondria may have the usual internal pattern of 



Fig. 7. Section of guinea pig epididymis transecting a group of sperm 

 tails at the level of the principal piece. Notice that the orientation of all 

 the cross sections is the same. This is attributable to the fact that the 

 broad flat sperm heads in this species stack up in rouleau in the third 

 part of the epididymis. This brings all the tails of the same group into 

 the same orientation. In this instance, presumably the flat heads would 

 be stacked in parallel array with their long transverse axes perpendicular 

 to the line (XY) through the central pair of fibrils. If we are correct in 

 assuming that XY is the dorsoventral axis and that the bending move- 

 ments are mainly transverse, then the tail is bilaterally asymmetrical, 

 having three dense fibers on one side and four on the other. In the guinea 

 pig and many other mammalian species the cross section of the tail is 

 symmetrical with respect to the axis passing through fiber 1 and between 5 

 and 6, but in some species (see Fig. 8) it is impossible to find a plane of 

 symmetry. 



Fig. 8. Cross section through the midpiece of a bat spermatozoon. The 

 mitochondria are flattened, semilunar in form, and of uniform size. There 

 are two to each turn of the mitochondrial sheath and they meet end to 

 end on or near the XY axis. This species is unusual in having four, in- 

 stead of three, members of the outer row of dense fibers that are larger 

 than the others and of very nearly the same size. These are numbers 1, 

 5, 6, and 9. No plane of bilateral symmetry can be drawn with this ar- 

 rangement of fibers. 



