PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 259 



such movement is seen in the large filaments on the surface of 

 the solution. During the formation of some of the larger fila- 

 ments having this motion, small spicule-like filaments grow out 

 radially from the edge of the pulsating membrane. One or 

 more forms at each pulsation until, when the filament ceases to 

 grow, the whole surface is covered with the small sharp filaments 

 and presents a bristly appearance. 



A similar rhythmic motion has been observed during the 

 formation of groups of small structures on the surface of nickel. 

 Here also the pulsating center is present, but small saccules, 

 about 30 to 60 microns in diameter, are formed around the 

 membrane instead of small filaments as in the case of zinc. 



The burr-like structures formed on copper have the same 

 rhythmic motion as the small structures which sometimes form 

 on iron. They exhibit both the pulsating movement of the 

 vesicle as a whole and the cilium-like motion of the separate 

 filaments. The rate of movement is usually about three per 

 second. 



Cadmium ferricyanide precipitation-structures have a more 

 regular rhythmic motion than in any other case so far observed. 

 The structures are large enough to enable the exact manner of 

 formation to be seen. Each structure originates at a minute 

 anodic region over which a small precipitation membrane is 

 formed, and the succeeding increase in the size of this membrane 

 takes place periodically. Growth is not all in the same direction ; 

 hence, characteristic shell-like structures are formed instead of 

 filaments (Fig. 18). The increase in size takes place through a 

 periodical rupture and reformation of the enclosing membrane. 

 At each rupture a slight outflow of solution occurs, but a new 

 membrane is at once formed, closing the opening; in this way 

 the structure is enlarged by the amount of new membrane formed 

 during the pulsation. When this increase in surface takes place 

 each time at a region whose position (relative to the metal) 

 remains unchanged, the structure formed is regular and sym- 

 metrical in shape. Sometimes, however, the rupture is first at 

 one region and an instant later at another; this tends to make 

 the structure irregular and asymmetrical in shape. The sur- 

 faces of the structures thus formed are finely striated, the striae 



