VARIATIONS IN SHAFT STRUCTURE — SPERM TAILS 43 



Peranema described on p. 39). Burgos and Fawcett observed that 

 the fin of Bufo is capable of the propagation of waves of movement 

 that can be independent of those of the main axis. 



The fin of Triturus differs in that the main axis of the tail is 

 formed by a very thick strand of fibrous material w^hich may be 

 horseshoe-shaped or trifoliate in section, and is accompanied for 

 part of its length by mitochondria. The axial fibre bundle runs 

 along the margin of the unthickened fin; movement of this 

 flagellum can cause undulations of the fin like those of the un- 

 dulating membrane of Trichomonas^ while the main axis of the tail 

 remains stiff (Fawcett, 1961). 



Sperm tails of mammals, both eutherian and marsupial, can be 

 divided into four regions : neck, mid-piece or middle-piece, 

 principal-piece and end-piece. The neck includes the basal 

 centrioles and forms a connexion between the head with its 

 nucleus and the mid-piece which is characterized by the presence 

 of a mitochondrial sheath. (The name mid-piece may also be used 

 in non-mammalian sperm for regions bearing a mitochondrial 

 sheath.) The principal-piece is surrounded by a more or less 

 spiral sheath of dense material, which may be fibrous or granular, 

 while the end-piece has no sheath, the fibril bundle being sur- 

 rounded only by the membrane which covers the whole of the 

 sperm. The relative lengths of the various regions differ in 

 different species, e.g. there is no end-piece in the bandicoot sperm 

 (Cleland and Rothschild, 1959), but the character of each region 

 is fairly constant. The excellent account of the structure of the 

 human sperm by Anberg (1957) will be used as an example for a 

 description of the various regions ; a set of diagrams by the same 

 author is reproduced in Fig. 10. 



The axial fibril bundle that runs the length of the tail has the 

 normal arrangement of tubular fibrils. Radial strands run from 

 the two centrals, which lie in a slightly denser region of the matrix, 

 to the nine peripherals, of which the two subfibrils can be seen to 

 be arranged circumferentially and not radially as Bradfield (1955) 

 suggested. Towards the tip of the end-piece the fibrils become 

 single and are irregularly scattered in the matrix. 



Outside the peripheral doublets, and on the same radii, is an 

 additional ring of nine coarse fibres of a homogeneous dense 

 composition, which extend from the neck through the mid-piece 



