558 BJORN A. AFZELIUS 



needle the cilium appears rigid if moved in one direction and limp if 

 moved in the opposite direction [4J. The force exerted on the water by 

 the cilium is in a plane perpendicular to its length. 



A flagellum is a fine vibratile thread projecting from a cell ; there are 

 normally only one or two flagella on a cell. The flagellar beat consists of 

 the formation of waves that propagate along the length of the flagellum — 

 either from the base to the tip or in the reverse direction. The water is 

 pushed along the length of the flagellum. In some cases it has been noted 

 that a defective flagellum is capable of forming stationary waves only, 

 these flagella will not propagate the water. 



There are many similarities between the flagellar beat and the ciliary 

 beat. In both cases the beat is in one plane, although successive beats of a 

 flagellum may be in planes that rotate along the length axis of the flagellum 

 [9]. In the cilium as well as in most flagella the beat can be described as a 

 bending movement starting at the base and transmitted to the tip. In the 

 flagellum the propagated waves follow each other closely and are fairly 

 symmetrical ; the flagellum might therefore at each instant take the shape 

 of a sine-wave (one wavelength long in the case of the sea urchin sperm 

 tail [9]). One implication of this, among others, is that the inner side of the 

 cilium may be shorter than the outer one at the end of the eiTective stroke ; 

 the two sides of the flagellum can retain their resting length throughout 

 the movements. It should be mentioned that cilia and flagella are active 

 units generating their own mechanical energy, they are not passively 

 moved by units within the cell body [8]. 



The comparatively simple movements performed by the cilia and the 

 flagella would not seem to require a very complicated type of machinery. 

 Therefore it has been astonishing to find that the machinery of cilia and 

 flagella is quite complicated indeed; the reasons for this are by no means 

 clear. 



Figure 5 (p. 562) is a cross-section through three sea urchin sperm 

 tails. This figure gives us a view into the motor units of the flagellum. The 

 appearance of the nine peripheral double filaments and the two central 

 ones has been described in an earlier communication [i]. The peripheral 

 filaments have projections called "arms" and "spokes", and these pro- 

 jections belong to one of the two subunits of the filaments. There is a 

 complex bridge formed between two of the peripheral filaments. The 

 filament opposite this pair is called the unpaired filament (filament i). 



The filaments of the sea urchin sperm tail are fixed proximally at a 

 disc in the basal bodv, and end freely distally as separated filaments at the 

 tip of the tail. 



Figures i and 2 are from another type of flagellum — the tail of the 

 squid spermatozoon. When the spermatozoon is actively swimming this 

 flagellum shows movements which are similar to those of the sea urchin 



