VARIATIONS IN SHAFT STRUCTURE — SPERM TAILS 47 



remaining two large fibres, which are those nearest to the plane 

 through the two central fibrils, lack these laminae, but they lie 

 close to the spiral sheath where it is formed on either side into 

 more or less continuous laminae which project inwards to make 

 close contact with the remaining peripheral fibrils of the axial 

 bundle. 



The spiral sheath in Perameles is composed of two parts, the 

 main part being a single-gyred spiral, which is tightly wound with 

 spaces of about 300 A between gyres. The gyres may split and 

 rejoin, so that their width is variable (500-3500 A); the thickness 

 of the sheath is more constant. The second part is the pair of 

 thickenings which occur in each gyre in the plane of the two 

 central fibrils. Each thickening, which is roughly triangular in 

 transverse section, contains two or three pairs of vacuoles em- 

 bedded in the sheath matrix, and carries a lamina which projects 

 inwards as mentioned above. Both parts of the sheath are made 

 up of a dense matrix containing randomly distributed granules 

 from 50 to 100 A in diameter. It can be seen from longitudinal 

 sections that there is continuity between the thickenings of 

 successive gyres only in the region which divides the pairs of 

 vacuoles and near the inner ends of the projecting laminae, i.e. 

 thin connecting strands are present in the plane of the central 

 fibrils on the minor axial plane of the tail. 



Around the spiral sheath is a number of concentric layers. Two 

 fairly dense layers 50 A thick separated by 50 A of less dense 

 material occur immediately outside the spiral sheath, and are in 

 turn surrounded by a thick (1000 A) layer of amorphous material 

 enclosed by the plasma membrane. Many structures are thus 

 contributing to the massive size of this sperm tail, whose diameter 

 is nearly 10 times that of the smallest cilium, although both have 

 an axial bundle of fibrils of the same size and arrangement forming 

 their central structure. 



The pattern of symmetry of the axial fibril bundle found by 

 Cleland and Rothschild in Perameles 6\S.qv^ in details from that 

 generally accepted by other authors as mentioned on p. 19. The 

 wide separation between the axial bundle and the large outer 

 fibres is unusual, as are the connecting laminae, although in other 

 sperm the thick outer fibres are very close to the peripherals of the 

 axial bundle so that connecting laminae are unnecessary. In many 



