60 STRUCTURE 



grew, and is concerned with centriole replication. It is possible 

 that centrioles or basal bodies that produce other structures from 

 this end lose the power of replication. A hub and spokes structure 

 has sometimes been seen at the centre of the proximal end of these 

 Viviparus centrioles, and in some procentrioles. 



Direct budding of the procentriole from the centriole has not 

 been seen, and it seems possible that the development of a pro- 

 centriole is " organized " or ** induced " near the proximal end 

 of the original centriole. The procentriole shows a ring of nine 

 structures right from the start, and, if this does not originate as a 

 thin slice from the end of an adult centriole, it is interesting to 

 speculate on the means by which such a complex structure could 

 develop. Bernhard and de Harven (1960) tentatively suggest that 

 a pericentriolar satellite (p. 49) might form the daughter centriole. 



A rather similar picture of the duplication of basal bodies has been 

 revealed by Grasse (1961) in Trypanosoma and Vickerman (1962) in 

 Blastocrithidia. Vickerman has found a short centriole lying at 

 right angles to the basal body of the functional flagellum of a 

 trypanosome; in other specimens tw^o full-sized centrioles may be 

 seen lying parallel to each other, which suggests that the short 

 centriole elongates and turns during its development, prior to the 

 growth of the flagellar shaft (PI. XIIc, d). Grasse also reports the 

 *' induction " of a new centriole near the basal body of the 

 flagellum. 



Mazia, Harris and Bibring (1960) have obtained some experi- 

 mental evidence that the critical time for centriole duplication in 

 sea urchin eggs is in the early interphase or even the preceeding 

 telophase, and we may conclude that it is at this time that the 

 initial formation of the procentriole takes place, either by fission 

 or by the organization of macromolecules. 



Both of the centriole functions mentioned involve the produc- 

 tion of protein structures, which, according to present theories, 

 are only synthesized in the presence of nucleic acids. The finding 

 of Randall and Jackson (1958) that kinetosomes of Stentor can be 

 stained by the Feulgen technique, the reports that basal bodies of 

 various cilia contain both DNA and RNA (Randall, 1959; Seaman, 

 1960), and many observations of granules similar to ribonucleo- 

 protein granules in basal bodies, as reported on p. 26, could all be 

 relevant to the synthetic ability of centrioles. 



