566 BJORN A. AFZELIUS 



the length of the cilium a contraction by sliding would seem possible only 

 if one of their ends is fixed and the other end free to move. The findings 

 presented here show that the free end might be either at the tip or at the 

 base of the cilium, or the flagellum. There is no correlation with the 

 direction of the propagated wave. As the nine peripheral filaments follow 

 straight paths [5, 15, 16] the filaments will have an unequal degree of 

 contraction (or sliding) in the uniplanar beat. In this connection it is of 

 particular interest to consider the possibility of lateral fusion of the 

 swimming-plate cilia by means of ridges from the "lateral" filaments. 

 These filaments would thereby be unable to contract or move during the 

 ciliary beat. 



The two central filaments are likely to be the candidates for the function 

 of an elastic backbone. Their position and morphology indicate that they 

 have this function and that they may determine the direction of the beat. 

 It has been shown that the inner filaments are in a line perpendicular to 

 the direction of the beat in mussel gill cilia [11], in ctenophore cilia [2], 

 and, as stated above, in the flagellum of the squid sperm. This is probably 

 also true of the choanocyte flimmerflagellum ; only when the flagellar beat 

 is perpendicular to the line through the inner filaments will the hairs be 

 helpful in increasing the efl^ective area of the flimmerflagellum. 



We are beginning to understand a little of the arrangement of the 

 filaments in cilia and flagella, but when it comes to an explanation of the 

 basic mechanism of ciliary (and flagellar) movement we are still left 

 without an answer. 



References 



1. Afzelius, B. A.,jf. biophys. biochem. Cytol. 5, 269 (i960). 



2. Afzelius, B. A.,_7. biophys. biochem. Cytol. (1961) (in press). 



3. Bishop, D. W., Nature, Lorid. 182, 1638 (1958). 



4. Carter, G. S., Proc. ray. Sac. B. 96, 115 (1924). 



5. Fawcett, D. W., and Porter, K. R.,^. Morphol. 94, 221 (1954). 



6. Gibbons, I. R., "Proceedings 2nd European Regional Conference Elect. 

 Micr., Delft," i960. 



7. Gibbons, I. R., and Grimstone, A.., J. biophys. biuchejti. Cytol. 7, 697 (i960). 



8. Gray, J.," Ciliary Movement". Cambridge University Press, Cambridge (1928). 



9. Gray, ].,y. exp. Biol. 32, 775 (1955). 



10. HoflFmann-Berling, H., Fortschr. Zool. Ii, 142 (1958). 



11. Huxley, H. E., and Hanson, ]., Ami. N.Y. Acad. Sci. 82, 403 (i960). 



12. Kilian, E. E., Z. vergl. Physio/. 34, 407 (1952). 



13. Manton, I., /// "Cellular Mechanisms in Differentiation and Growth", ed. 

 D. Rudnick. Princeton University Press, Princeton (1956). 



14. Nagano, 'V.,jf. appl. Pliys. (in press). 



15. Rhodin, J., and Dalhamn, T., Z. Zellforsch. 44, 345 (1956). 



16. Sjostrand, E. S., and Afzelius, B. A., "Proceedings ist European Regional 

 Conference Elect. Micr., Stockhf)lm". Almcjvist and Wiksell, 164 (1956). 



