THE FORM OF BEAT OF CILIA 



139 



B 



C 



D 



Fig. 36 Diagram illustrating the supposed transition from a 

 symmetrical flagellar beat to an asymmetrical ciliary beat. The 

 outlines in A^ B and C show the successive shapes and positions 

 of sea urchin sperm tails relative to their heads, which are 

 drawn as if they were fixed; in the movement of these sperm 

 the heads of course moved from side to side. A represents a 

 cycle of beat of a symmetrically beating sperm tail, in B the 

 cycle is moderately asymmetrical "and in C it is very asymme- 

 trical. D represents a cycle of beat of a membranelle of Stentor. 

 (Sea urchin sperm outlines drawn from Gray, 1955; A and B 

 were taken from outlines shown in Fig. 31.) 



