THE MECHANISM OF LIFE 571 



lying tissue be excised from a Beroe, a preparation is obtained in 

 a form easy to study. The movement begins in the uppermost 

 plate and proceeds to all the succeeding ones. If the uppermost 

 plate is at rest, so are all the succeeding ones ; a plate in the 

 middle of the row never contracts while the preceding ones are at 

 rest. If a plate in the middle of the row be held fast, the ciliary 

 waves course from above down to this plate only ; they stop here, 

 and all the lower plates in the series stand still (Fig. 274, ///). 

 Thus every plate is in the closest dependence upon the one next 

 above, and is never able to move independently. But the move- 

 ment of all the plates is determined in this manner by the first 

 plate of the series (Fig. 275, A). In spite of this, every plate 

 possesses in potentia a certain independence. If, e.g., the row be 

 cut through, the plate that now stands first in the series under- 

 takes the lead, and controls by its contraction and its rest all the 

 plates standing below it, so that the two separated halves of the 



B 



FIG. 275. .4, Intact row of cilia showing normal metachronism of beat. The uppermost (left) 

 cilium inaugurates the rhythm and the others follow at the same rate. B, Row of cilia divided 

 in the middle by a cut. Each half acts with its own rhythm. 



row now act with separate rhythms (Fig. 275, B}. Every indi- 

 vidual plate, indeed, taken out of the series contracts rhythmi- 

 cally, provided that the cell-body belonging to it is still present. 

 There is here an interesting case of complete subordination. Every 

 ciliated cell of an epithelium, so long as it is living, possesses in 

 itself complete autonomy as regards its movement ; in union with 

 its like, however, it has wholly given up the independence of its 

 movement. This is necessary if a metachronous motion, which 

 possesses essential advantages, is to come about. The same relation 

 is found not only between the individual ciliated cells of an epithe- 

 lium, but also between the individual cilia of a cell. In the latter 

 the same metachronism of beat exists in a long row of cilia, as may 

 be seen especially plainly in ciliate-infusorian cells. No cilium acts 

 before the one preceding it in the row. If the uppermost one 

 rests, the whole series is quiet. Nevertheless, every individual 

 cilium, separated from the others, shows complete independence of 

 movement. If, e.g., in Spirostomum the long row of peristome 



