A TYPICAL ORGANELLE TRANSITION ZONE 25 



taking place in the transition zone will be described as they appear 

 in transverse sections at successively lower levels of the cilium, i.e. 

 as in diagrams A to G of Fig. 7. 



The first change at the outer end of the transition zone is the 

 appearance of links between the peripheral fibrils and the ciliary 

 membrane (Fig. 7B). At the same level some of the arms may 

 become less obvious and an additional bridge-link is developed 

 between fibrils 1 and 2. This structure continues for about half 

 of the length of the transition zone before an abrupt ending of 

 radial links and secondary filaments, and the appearance of 

 similar bridge-links between all of the peripheral fibrils (Fig. 7C). 

 In addition, a ring of 18 (probably) dense dots appears just inside 

 the ring of peripheral fibrils. A little below this the two central 

 fibrils and associated structures end, and the diameter of the 

 ciliary membrane is decreased to form a tight collar around the 

 axial bundle of fibrils (Fig. 7H). A pair of plates run across the 

 cilium at this level and separate the contents of the shaft from the 

 basal body cavity; they form a fairly dense region around the 

 peripheral fibrils. Below these basal plates the diameter of the 

 ciliary membrane increases once more, and the peripheral doublets, 

 which retain connexions to the membrane and to each other, are 

 shown to be tilted further (Fig. 7E). Connexions between all the 

 A subfibrils may develop and the A-B connexions disappear before 

 the inner end of the transition region is reached. The C subfiurils 

 appear at about the level of the ceil surface ; at their distal end each 

 one is associated with two fine transitional filaments, which run 

 to the cell membrane (Figs. 7F and 7H). The triplets of the basal 

 body run inwards from this level and are now tilted at an angle 

 of about 40° to the tangent to the ring of fibrils. 



Gibbons points out that the significance of this complexity is 

 obscure, as is also the connexion between the structures found at the 

 different levels. He stresses the problems of the development of 

 such a complex structure by outgrowth from the cell. The transi- 

 tion zone in Anodonta occupies the thickness of the brush border 

 of the cells; the presence of the brush border may influence the 

 development of the cilia. 



It is not known whether other cilia have complex transition 

 zones like these, but we have no reason to doubt it. Further 

 studies may reveal a variety of individual patterns or some com- 



