ELEMENTARY VITAL PHENOMENA 



251 



individual cilia can be followed in Infusoria under the microscope, 

 if the stroke be slowed by placing the objects in a thickish 

 medium, such as a solution of gelatine. It is then found that the 

 rest ing-position, from which the cilium performs its movements, 

 is changeable. At one time the cilium lies more against the body r 

 at another time it stands more vertical ; hence the amplitude of the 

 swing, and thus the amount of the motor effect can be very finely 

 graded (Fig. 111). 



It follows from the change of form of the individual cilium in 

 carrying out the stroke, that in the progressive phase a contraction, 

 starting from the base of the cilium, takes place on the side 

 toward which the stroke is carried out, for a simple measurement 

 shows that this side is shortened when it passes into the position 

 of extreme swing. At the same time the opposite side is drawn 



FIG. 111. Movement of a single cilium of a ciliate infusoriau (Urostyla grandis, border-cilium) 

 from two different resting-positions, / and 11. A. Progressive, B, retrogressive phase of the 

 movement in several successive stages. The arrows indicate the direction toward which 

 the body is driven. 



over passively, being extended necessarily, according to simple 

 mechanical principles. In the retrogressive phase the contracted 

 side relaxes, and to the same extent the cilium, as a result of the 

 elasticity of the extended side, bends back into the position of 

 rest. The progressive phase, therefore, is the phase of contraction, 

 the retrogressive phase that of expansion of the single stroke of 

 the cilium. The play of the ciliary movement comes about by the 

 rhythmic alternation of the two. 



But all cilia do not contract in one plane like those of the 

 swimming-plates of the ctenophores. Many, especially certain 

 flagella, describe more complicated paths, funnel-shaped, screw- 

 shaped, and like the path of a whip-lash, and accordingly the 

 earlier physiologists distinguished several forms of ciliary move- 

 ment. But whatever the path of vibration of the different cilia 

 may be, the same principle lies at the basis of all, viz. : that a 



