NEUROID TRANSMISSION 71 



epithelium are somewhat like the neurones in the differen- 

 tiated central nervous organs in that transmission from 

 element to element is easily accomplished in one direc- 

 tion hut not in the reverse. Hence this type of transmis- 

 sion is definitely restricted in its course. That this is a 

 structural feature of ciliated epithelium is shown in the 

 experiment of von Briicke (1916) who demonstrated that 

 when a small piece of ciliated epithelium was cut out and 

 reimplanted after having been turned through a half- 

 circle, it retained after healing the original direction of 

 its effective stroke and its transmission wave was in op- 

 position to that of the surrounding field. Thus whatever 

 it is that determines the polarization of ciliated epithelia, 

 that feature is resident locally in the epithelium itself 

 and is not impressed upon it from some external source. 



The facts that have thus far been brought out for cil- 

 iated epithelium can also be demonstrated for the most 

 part on the rows of swimming plates on ctenophores. In 

 these animals each swimming plate is like a gigantic flat- 

 tened cilium. It is compound in nature, for it arises from 

 a group of cells instead of from a single cell ; otherwise, 

 it is essentially like a cilium. These plates, as already in- 

 dicated, form eight well-defined rows, extending over the 

 body of the ctenophore from its aboral toward its oral 

 pole. Transmission waves pass over these rows in an 

 oral direction, but the effective stroke of each plate is in 

 the aboral direction, thus driving the ctenophore through 

 the water mouth foremost. The rows of plates, therefore, 

 like rows of cilia, exhibit a well-developed metachronism. 

 The fact that the plates are often 'normally quiescent and 

 may become active only at intervals makes them very 

 convenient for experimental work. 



If the longitudinal band of tissue to which the plates 



