42 STRUCTURE 



membrane (PI. Xlb) (Rothschild, 1955). An additional ring of 

 nine outer dense fibrils is present immediately outside the peri- 

 pheral doublets in the mid-piece of sperm of the snail Helix 

 (Grasse, Carasso and Favard, 1956). This mid-piece carries a 

 sheath of several concentric layers of membranes inside the 

 mitochondrial helix, and a sheath of longitudinal fibrils (200 A 

 in diameter) around the outside. 



A modification of the central part of the axial fibril bundle has 

 been found in the sperm tail of the flatworm Haematoloechus by 

 Shapiro, Hershenov and Tulloch (1961). Here the usual pair of 

 fibrils is replaced by a single structure made up of three parts, 

 which have an overall diameter of 500 A. At the centre is a dense 

 core surrounded by a less dense cortical region, around which is a 

 sheath with some indications of a fibrillar structure. The authors 

 believe that there is a connexion between spiral striations of this 

 sheath and the prominent radial spokes running from the central 

 structure to the nine normal peripheral doublets (PI. Xb, c). 

 The spokes are found to occur at intervals of about 220 A along 

 the axis. Another interesting feature of this sperm tail is the 

 sheath which is said to be made up of 36 longitudinal fibrils (about 

 200 A in diameter), but these were not present in all of the micro- 

 graphs presented and may enclose only a specialized part of the 

 tail as in Helix. 



Most of the vertebrate sperm studied have been obtained 

 from mammals, but some important variants of structure have 

 been reported from other vertebrates, particularly from the 

 amphibians, while fish sperm (Lowmann, 1953) and bird sperm 

 (Grigg and Hodge, 1949) seem to follow the normal pattern. The 

 sperm tails of the toad Bufo (Burgos and Fawcett, 1956) and the 

 newt Triturus (Fawcett, 1958b; 1961) both show a large lateral 

 fin running the length of the tail in the plane which passes through 

 the two central fibrils of the axial bundle. In Bufo this fin is 

 composed of a thin layer of fibrous material which extends from 

 a thickened strand that runs alongside the axial fibril bundle to 

 another thick strand that runs along the outer edge of the fin, the 

 whole being surrounded by the tail membrane (PI. XIa). The 

 maximum width of the fin is about 1 /jl and it narrows towards the 

 base of the tail, where the fibrous material is said to have its 

 origin at a second centriole (cf. the intraflagellar material in 



