A TYPICAL ORGANELLE — SHAFT 19 



1600 A. Each of these fibrils is double, as shown in Figs. 4b and 5, 

 being composed of two subfibrils which appear in transverse 

 sections to be tubular, with a wall about 60 A thick and a circular 

 or elliptical outline of diameter about 200 to 250 A (see Table 3). 

 The dividing wall between the two subfibrils is also about 60 A 

 thick. Recent electron micrographs, particularly those of Afzelius 

 (1959, 1961a) (PL Xa and Vb), Gibbons and Grimstone (1960) 

 (PL 1) and Gibbons (1961b) (PL lid), have shown the presence 

 of two longitudinal rows of *' arms " on one of the subfibrils 

 (labelled A in the figures) of each peripheral doublet. These arms 

 can be seen in both transverse and longitudinal sections (Fig. 7) as 

 rods about 120 to 150 A long and 50 A thick, which are spaced 

 about 130 A between centres in longitudinal rows, and which 

 project perpendicular to the axis of the shaft towards the adjacent 

 peripheral doublet. Gibbons (1961a) has found that in all the 

 cases he examined the arms project in a clockwise direction, as 

 seen by an observer looking from the base. Subfibril A, which 

 bears the arms, often has a denser core than the other subfibril ; in 

 at least some cases this is because one of the arms extends back 

 across the subfibril, as seen in Fig. 5a. This subfibril has some- 

 times been found to be larger than subfibril B (Fig. 5b). In 

 echinoderm sperm tails (Afzelius, 1959) and Anodonta cilia 

 (Gibbons, 1961b), arms are present on subfibril B of peripheral 

 fibril number 6 ; these link with those of the A subfibril on fibril 5 

 to form a complex bridge, from which a link runs to the ciliary 

 membrane in Anodonta cilia (Fig. 5a). Gibbons and Grimstone 

 (1960) and Lansing and Lamy (1961a) have found some indications 

 of a helical substructure in these peripheral fibrils ; the latter authors 

 found a helical strand 40 A thick with a 140 A pitch, which may 

 be associated with the arms. 



Bradfield (1955) postulated that the nine peripheral fibrils are 

 spaced at equal distances from adjacent fibrils and also at equal 

 distances from the centre of the cilium. He also suggested that 

 the fibril arrangement is symmetrical about a line at right angles to 

 the line through the central fibrils, as shown in Fig. 6a. Most 

 authors have agreed with these hypotheses, but first Cleland and 

 Rothschild (1959) and then Lansing and Lamy (1961a) have chal- 

 lenged both of them. These two pairs of workers have found that 

 the fibrils were not equally spaced from their neighbours, some 



