CILIARY REVERSAL 195 



depolarization. Naitoh (1958) found that the membrane showed 

 a difference in sensitivity at different parts of the body surface; 

 this was at least partly attributable to differences in membrane 

 resistance, so that the same amount of depolarization may give 

 different degrees of change in beat direction at different parts of 

 the body surface as in Fig. 53 (especially Stage IV ventral). 

 This depends on the assumption that the depolarization spreads 

 over the whole cell without appreciable decrement. It is 

 interesting that the direction of beat changes apparently simul- 

 taneously over the whole body of Opalina, and further, Tartar 

 (personal communication) has found that where several stentors 

 have been grafted together, body cilia over the whole surface of 

 the joined mass reverse simultaneously. Incidentally, the beat 

 of the membranelles is inhibited at the same instant. A rapid 

 spread of electrical depolarization over the surface of these 

 protozoa is thus held to cause a change in the direction of beat, 

 and this would explain why the reversal response affects the whole 

 surface of an animal like Paramecium even when parts of the body 

 have been almost isolated by cuts. 



It is also interesting to speculate on the mechanism of reversal 

 in these protozoa. As Kitching suggests, there may be a parallel 

 between the relationship of depolarization to ciliary reversal and 

 the relationship of depolarization to muscle contraction, and it is 

 tempting to suggest that in both cases ions, particularly potassium 

 ions, are intimately involved. The experimental production of 

 reversal involves treatments which either upset the balance between 

 monovalent and multivalent cations, or upset the stability of the 

 membrane electrically or mechanically. Either treatment could 

 permit a change in the pattern of ionic movements across the 

 membrane. While an increased concentration of monovalent 

 cations outside the body will induce reversal and divalent ions 

 will not, the excitability for this reversal is reduced in the absence 

 of calcium and increased in calcium -rich media (Kinosita, 1954; 

 Okajima, 1954b). The fact that the injection of calcium precipi- 

 tants induces prolonged reversal of beat in Opalina (Ueda, 

 1956), may be another result of ionic unbalance or may have a 

 more specific effect in that reduction of internal calcium 

 concentration induces reversal. It seems possible, then, that the 

 membrane depolarization involved in reversal permits ionic 



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