SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 165 



opposite sides of the fibrous core of the midpiece (Fig. 8). So uni- 

 form is the size of the mitochondria and so regular is their arrange- 

 ment that their end-to-end junctions in the successive gyres form 

 two straight lines on opposite sides of the middle piece. Moreover, 

 it can be shown in transverse sections of this segment that the lines 

 of end-to-end junction of the mitochondria nearly coincide with the 

 plane of the central pair of fibrils in the axial filament complex 

 (Fig. 8). The line of mitochondrial junctions in the midpiece of the 

 bat sperm therefore constitutes a surface landmark which is a useful 

 guide to the orientation of the internal fibrils. Thus in oblique sec- 

 tions that pass through the head parallel to its transverse axis and in- 

 clude the first part of the midpiece (Fig. 12), it can be ascertained 

 that the line of junctions of the mitochondria (and also the central 

 pair of fibrils) is in the dorsoventral axis of the spermatozoon. If it 

 be assumed that the plane of bending movements of the tail is at right 

 angles to the plane of the central pair of fibrils, as is true of cilia, then 

 it can be concluded that the tail movements in this species are mainly 

 in the transverse plane. The same may prove to be true of other 

 mammalian sperm whose heads are flattened dorsoventrally. Similar 

 observations on oblique sections at the junction of the middle piece 

 and principal piece reveal that the opposite longitudinal compo- 

 nents of the fibrous sheath are on or near the dorsoventral axis of 

 the spermatozoon. 



When fresh bat sperm are examined with the phase contrast mi- 

 croscope, it is possible, where sperm heads present their broad face, 

 to make out a thin luminous line along the middle piece which ap- 

 parently represents the line of junction of the mitochondria. The 

 presence of a visible surface landmark in the middle piece that is 

 related to the orientation of the internal fibrils may make bat sperm 

 particularly favorable material for future studies correlating cine- 

 matographic analysis of the tail movements with the fine structure 

 and arrangement of their internal fibrous components. 



CONCLUDING COMMENT 



The objective of comparative electron microscopic studies of sper- 

 matozoa is to discover which structural features are common to all 

 mammalian species, and which are peculiar to one or to only a few. 



