68 THE ELEMENTARY NERVOUS SYSTEM 



stroke is in the same direction as the general wave, it is 

 not so easily applied to cases where, as in the cteno- 

 phores, the reverse is true, that is, the effective stroke is 

 in a direction opposite to that of the wave of ciliary ac- 

 tion. Here the effective stroke of a plate would not be 

 directed toward the one next to act, but toward the one 

 that had just previously acted. Hence the mechanical 

 explanation offered above fails of satisfactory applica- 

 tion. For reasons that will be given presently this 

 mechanical explanation can also be shown to be entirely 

 inapplicable also to those cases in which the direction of 

 the effective stroke and that of the ciliary wave agree. 



If the wave of ciliary metachronism is not due to the 

 direct mechanical action of one cilium on another, the 

 form of coordination that this process exhibits must de- 

 pend upon some part of the cell deeper than the ciliated 

 zone. Such might well be the cytoplasm, even the super- 

 ficial cytoplasm, of the cell itself. Evidence in favor of 

 this opinion was advanced by Kraft as early as 1890. 



Kraft showed by two methods of experimentation that 

 stimuli applied to a ciliated field on one side of a band 

 of quiescent cilia could influence ciliary action on the 

 other side of this band, thus demonstrating a transmis- 

 sion over a region devoid for the time being of any form 

 of mechanical activity. The first method of experimenta- 

 tion involved the use of a small temperature box divided 

 into three chambers and so arranged with inlets and out- 

 lets that each chamber could be supplied with a current 

 of water of fixed temperature (Fig. 16). These cham- 

 bers all abutted on one face of the box and thus this face 

 represented a surface on which there might be established 

 three sharply defined areas each with its own tempera- 

 ture. The whole apparatus was of such small size that 



