154 DON W. FAWCETT 



over, in subtangential longitudinal sections through one of the thick- 

 ened parts of the sheath (Fig. 16), one can see very clearly where the 

 ribs join a continuous longitudinal band. The size of this longi- 

 tudinal component varies with the species but its reality is unde- 

 niable, even in the bat and in the human sperm tail where the thick- 

 enings of the fibrous sheath are much less conspicuous than they are 

 in the rodents. In median longitudinal sections of the principal piece 

 (Fig. 14), the ribs of the sheath are seen in cross section and display 

 a pleomorphism entirely inconsistent with the older interpretation 

 of the fibrous sheath as a simple helical wrapping. 



Very little is known about the mode of formation, the chemical 

 nature, or the functional significance of the fibrous sheath. Studies 

 of its solubility properties indicate that it is a highly resistant sclero- 

 protein (Bradfield, 1955). It is probably not directly concerned with 

 producing the tail movements but may be an important supporting 

 or elastic component of the tail. It has been suggested that it may 

 have a springlike function, offering resistance to the waves of con- 

 traction that pass along the axial filament complex, thus providing 

 the necessary couple for the transverse bending movements of the 

 tail (Challice, 1953). 



ORIENTATION OF TAIL COMPONENTS WITH RESPECT TO THE 

 PLANE OF TAIL MOVEMENT 



It has been established that cilia beat in a direction perpendicular 

 to the line through the centers of the central pair of internal fibrils 

 (Fawcett and Porter, 1954). Lack of fixed landmarks makes it more 

 difficult to relate the internal structure of the sperm tail to the plane 

 of its beat. In some species, however, the flattened shape of the head 

 makes it possible to assign dorsoventral and transverse axes. From 

 suitably oriented thin sections, one can then determine the position 

 of middle-piece structures in relation to these axes. We have under- 

 taken such a study in the bat sperm which has a structure that is 

 particularly favorable for such an analysis. 



The head of the bat sperm is somewhat flattened and the longer 

 dimension of its elliptical cross section is designated the transverse 

 axis and the shorter, the dorsoventral axis. The mitochondrial sheath 

 is unusual in that the pitch of the helix is very flat and each gyre is 

 made up of two crescentric mitochondria of equal size that meet on 



