STRUCTURE AND FUNCTION IN AMOEBOID MOVEMENT 555 



be quite compatible with this front contraction theory. For example, it has 

 been pointed out that occasionally the endoplasmic stream splits longi- 

 tudinally, and portions of the stream move in opposite directions as if 

 pulled by opposing fountain zones [2, 3]. The reversal of streaming also 

 occurs as if most of the endoplasm were pulled instead of pushed in its 

 new direction, for reversal begins first at the new advancing front and 

 stops last at the old advancing front [3, 4]. Each front exhibits normal 

 hyaline cap production cycles throughout the change. While it may be 

 possible by means of additional assumptions to reconcile these facts with 

 the tail contraction theory, it is important to realize that these observations 

 are fulfilments of the predictions one would make from the fountain zone 

 contraction theory even if one had never seen an amoeba. 



The fountain zone contraction theory is only the first step toward the 

 localization, identification, and understanding of the molecular mechanism 

 of amoeboid movement. We can hope that the "engine" of amoeboid 

 cells, once localized, will be easier to disect and characterize by physical 

 and chemical experiments. As has been pointed out elsewhere [3, 4], the 

 principle behind the theory mav have wider applications to other systems 

 the mechanisms of which have been obscure, such as reticulopodial 

 movement in foraminifera and certain cases of protoplasmic streaming 

 in plants. 



References 



1. Allen, R. D., Bidl. Bull. 109, 339 (1955). 



2. Allen, R. D., 7- biuphys. biuclum. Cytol. 8, 379 (i960). 



3. Allen, R. D., /;/ "The Cell", ed. J. Brachet and A. E. Mirsky. Academic Press, 

 Xew York and London (in press). 



4. Allen, R. D., Exp. Cell Res. Sitppl. (1961) (in press). 



5. Allen, R. D., Cooledge, J., and Hall, P. J., Nature, Loud. 187, 896 (i960). 



6. Allen, R. D., and Roslansky, J. D.,;7. biuphys. biochem. Cytol. 4, 517 (1958). 



7. Allen, R. D., and Roslansky, J. D., J. biophys. biochem. Cytol. 6, 437 (1959). 



8. Dellinger, O. P., J. e.\p. Zool. 3, 337 (1906). 



9. Dujardin, F., Aim. Sci. uat. Zool. 4, 343 (1835). 



10. Griffin, J. L., and Allen, R. D., Exp. Cell Res. 20, 619 ( i960). 



11. Cioldacre, R. J., and Lorch, I. J., Nature, Loud. 166, 497 (1950). 



12. Heilbrunn, L. V., Protnplasuia 8, 65 (1929). 



13. Kamiya, X., and Kuroda, K., Bot. Mag., Tokyo 69, 544 (1956). 



14. Mast, S. 0.,y. Morph. 41, 347 (1926). 



15. Mast, S. O., and Prosscr, C. I.., J. cell, couip. P/iysiol. i, t,33 {^93^)- 



Discussion 



CJoldacre: How w(juld you account on your hypothesis for the fact that ATP 

 injected into the cell causes a local contraction at the site of injection which then 

 becomes the tail, not the front ? The second tjuestion : I gather that your hypothesis 

 rcc]uires a propagated contraction which is held in place by the U-shaped bend at 



