Cilia 647 



lium in one direction in a regular sequence (Fig. 245). Each cilium is very 

 slightly out of phase with its neighbor in front of and the one behind it. 

 This fact, together with the fact that the cilia at right angles to this wave 

 of activity are beating in phase with one another, gives the visual impres- 

 sion of waves passing over the epithelium. The crests of the waves include 

 those cilia at the peak of their effective stroke; the troughs include those 

 cilia' at the ends of their effective strokes and about to commence their re- 

 covery. 



The direction of the mctachronic wave in a ciliated surface appears to be 

 as fundamental a property of the surface as the direction of ciliary beat. 

 Even a small isolated portion of a ciliated surface continues to show its own 

 inherent direction of metachronism. This is not disturbed by removing and 

 then replacing after rotating through 180° a portion of the ciliated epithe- 

 lium of the roof of a frog's mouth.'* In these circumstances the transplanted 

 portion has its metachronic wave proceeding in a direction opposite to that 

 in the surrounding tissue. 



-> •ff«ctiv« bMk 



|1|2|3|4| 9|»|7|S|9|lO| II ll^l 

 .f 1 L. ^ 



r«covefy stroke sf fl»cliv« strok* 



Fig. 245. Diagrammatic representation of the metachronal rhythm. 

 Cilia 1 to 12 are beating in sequence. From Gray."* 



Among ciliated tissues from various sources, the direction of the meta- 

 chronic wave bears no constant relationship to the direction of the effective 

 stroke. In some, as in the frog's mouth, the metachronic wave and ciliary 

 effective strokes are in the same direction. In the rows of ciliated combs of 

 ctenophores the two are in opposite directions.^® •''•*'• In the cilia of the gills of 

 the annelid, Nephthys,^^ and in the lateral ciliated epithelium of Mytilus 

 gills-*^ the metachronic wave is always at right angles to the effective stroke. 

 There is no essential fixed relationship between the two. 



Little is known of the mechanisms of the coordination and control of 

 cilia. There is reason to believe that it is more complex among protozoans, 

 in which a number of complex locomotory patterns are exhibited by a single 

 individual, than in the ciliated epithelia of the larger metazoan invertebrates 

 and vertebrates. In these higher animals the pattern of coordination seems 

 for the most part to be a highly invariable one. Among the protozoans there 

 appear to be definite species differences in the pathways of the metachronic 

 wave. This is not interfered with by a transverse cut deep into the animal 

 in Spirostomutn and Stentor, but coordination on the two sides of such an 

 incision is lost in Paramecium.^^ Transmission of the metachronic wave 

 would appear to be by way of a conducting network in the ectoplasm, per- 

 mitting passage in all directions through the ectoplasm in the former species 

 and only along ectoplasmic, longitudinal pathways in the latter. In view of 

 the fact that the silver-line systems of ectoplasmic fibrils in the first two 

 species show the form of a network connecting the basal granules, and in 

 the latter species possess the form of only longitudinal fibrils, it has been 

 suggested that this fibrillar system forms the conducting pathway for the 

 metachronic wave in protozoans. 



