26 STRUCTURE 



mon features of structure. The transition zone is obviously an 

 important part of the ciHum, for it Hnks the basal region, respons- 

 ible for excitation and anchorage, to the contractile shaft. It is 

 interesting that connexions between the peripheral fibrils and the 

 ciliary membrane in the transition zone have been widely found, 

 and these may always occur, e.g. in the connecting cilium of the 

 mammalian retinal rod (Tokuyasu and Yamada, 1959) and the 

 sperm of Hydroides (Colwin and Colwin, 1961), as well as in some 

 cilia and flagella mentioned above. 



The Basal Body. 



The kinetosome or basal granule of ciliated protozoa, the 

 the blepharoplast (or only part of this, according to some authors) 

 in flagellated protozoa, and the centriole, or one of the centrioles, 

 at the base of the flagellum of sperm, are the basal bodies of the 

 ciliary organelles that these cells carry. The similarity of the basal 

 body to the centriole present in many animal cells, and the 

 derivation of the basal body from the centriole will be discussed 

 later (see p. 48). 



The basal body is a cylindrical structure, usually 4000 to 5000A 

 long and 1500 to 2000 A in diameter (see Table 4, p. 53) with a 

 less dense central region and a dense wall containing the con- 

 tinuations of the peripheral fibrils of the shaft together with 

 intercalary matter which links the fibrils into a continuous 

 cylinder. The outer end of the basal body is continuous with the 

 ciliary shaft, while the inner end may be open and continuous with 

 the cell cytoplasm, or closed by opaque material which is usually 

 connected to some sort of root structure. Fibres in the cell 

 cytoplasm may be connected to any part of the basal body, most 

 frequently at the- inner end, and quite often also near the middle ; 

 in both places the wall of the cylinder may be thickened at the 

 fibre attachments (see PL IVb). 



The central region of the basal body is seldom completely 

 structureless. Granules about 200 A in diameter have often been 

 observed and are thought to have a connexion with the reputed 

 duplication and fibril production properties of the basal body. 

 Randall and Jackson (1958) report that kinetosomes of Stentor are 

 stained by the Feulgen technique, and that the internal granules 

 are most frequent in mitotic or conjugating animals, while 



