210 THE ORIGIN OF THE NERVOUS SYSTEM 



kind, and some sort of transmission must be concerned 

 in the progress of the wave of beats over the surface. 

 These ciliated surfaces occur in many animals and in 

 stages of development where no distinguishable nervous 

 structure exists, and even in cases in which a well- 

 developed nervous system is present it has not been 

 generally demonstrated that ciliary movement is under 

 nervous control, although Putter (1903) states that in 

 certain animals such control does occur. Verworn 

 (1890) maintained that transmission in such cases is 

 purely mechanical, each cilium in its movement touch- 

 ing the next and so exciting it. Kraft (1890) and 

 others have shown, however, that Verworn's view is 

 incorrect, since transmission of the impulse may occur 

 through a region in which the actual movement of the 

 cilia has been completely inhibited by cold and after 

 passing through such a region is still able to excite the 

 cilia beyond. Briicke (19 16) also found that when 

 pieces of ciliated epithelium were cut out and reim- 

 planted in the same spot in reversed position with 

 respect to the direction of transmission, they showed a 

 direction of transmission opposite to that in other 

 regions of the epithelium. Such pieces evidently retain 

 the original direction of transmission quite independ- 

 ently of any mechanical action of the cilia of adjoining 

 regions. Mechanical transmission may perhaps occur 

 in some cases of ciliary activity, but it is certainly not 

 the fundamental factor in the passage of impulses over 

 ciliated surfaces. The impulse is evidently an excita- 

 tion of some kind and its transmission a protoplasmic 

 process, but no definite conducting path is distinguish- 

 able. Parker (1905) has called transmission of this 



