VARIATIONS IN SHAFT STRUCTURE — SPERM TAILS 41 



are present (Moses, 1961). The basic pattern of the 9-plus-2 axial 

 fibril bundle has been found in the tails of most sperm examined, 

 but is nearly always supplemented in some way for at least part of 

 the length of the tail. Mitochondria are very often found in the tail, 

 either as a sheath surrounding it for a greater or lesser part of its 

 length, or as strands running along the tail. These mitochondria are 

 usually modified during development and lose the usual arrange- 

 ment of internal cristae, e.g. in the rat sperm (Yasuzumi, 1956); 

 this may allow the storage of a large quantity of substances with 

 readily available energy. Similar modifications occur in the fern 

 Pterldium, where the mitochondria are retained within the body 

 of the multifiagellate spermatozoid (Manton, 1959b). Another 

 sheath of a fibrous or granular material is found surrounding the 

 tail for much of its length, especially in mammals, where an 

 additional set of nine fibres outside the peripherals is also found 

 in the proximal part of the tail. The sperm tail of the ostracod 

 does not follow the usual plan (Bradfield, 1955), and deserves 

 further study. 



Afzelius (1959) found that the tail of a sea urchin sperm is 

 structurally an unmodified cilium. At the base is a cylinder of 

 fibril triplets, and the shaft has two central fibrils with radiating 

 strands to the nine peripheral doublets, each of which carries 

 transverse arms on one subfibril ; the whole shaft is surrounded by 

 a three-layered membrane (PI. Xa). Around the base of the tail 

 is a mitochondrial sheath about 0-5 /x long and 1 /x in diameter, 

 which fits against the nucleus to form the posterior part of the 

 sperm head (Rothschild, 1956). The sperm of the annelid 

 Hydroides is very similar in the structure of the tail, with two 

 centrioles and a cluster of mitochondria (Colwin and Colwin, 

 1961). 



Other invertebrates have sperm tails which may follow a 

 similar simple plan with various patterns of distribution of 

 mitochondria. Helical mitochondrial strands are present around 

 the fibril bundle in slugs and snails, while the sperm tails of the 

 cockroach Periplaneta and the bee Apis carry a pair of straight 

 longitudinal strands of mitochondria (Bradfield, 1955). The 

 fibril bundle of the sperm tail of the bee, which appears to have 

 enlarged and modified peripheral fibrils, is also accompanied by 

 a pair of additional strands of uncertain structure inside the tail 



