STRUCTURES RESEMBLING ORGANIC GROWTHS. 139 



of the thicker filaments may be several centimeters long, usually 

 extending in a straight or sinuous course over the surface of the 

 solution. They are hollow, unbranched, of smooth contour, and 

 exhibit considerable coherence and flexibility; these properties, 

 however, as well as the rate of growth, vary with the content 

 of the solution in albumin and ferricyanide. The rate of forma- 

 tion also varies with the character of the metal, being in general 

 slower with pure than impure specimens of iron; it is also 

 greatly influenced by the contact of other metals, as will be 

 described in detail below. 



The actual process of formation presents a striking spectacle 

 under low powers of the microscope. Within a few seconds the 

 precipitate appears upon the surface of the metal in scattered 

 regions, at first as minute rounded vesicles each enclosed by a 

 thin precipitation-membrane; slender cilia-like filaments then 

 push out rapidly into the solution; these steadily elongate, many 

 reaching in a few seconds a length of 50 microns or more. In 

 most cases growth ceases abruptly at a length of a small fraction 

 of a millimeter; this is usually the case with the more slender 

 formations; the broader filaments may continue to elongate by 

 terminal growth until they reach in many cases a length of several 

 centimeters. At their earliest appearance the filaments strongly 

 suggest organic growths like cilia, and what is still more remark- 

 able is the fact that they often exhibit trembling or wavy or in 

 some cases quite regularly rhythmical to-and-fro vibratory move- 

 ments. This phenomenon is also observed with the similar 

 filamentous growths of zinc and copper ferricyanide, especially 

 the latter, and will be considered further below. Since in living 

 cells cilia typically arise from locally modified regions of the 

 surface protoplasm ("basal bodies") which serve as centers of 

 rhythmical activity as well as of growth, this behavior of newly 

 formed precipitation-filaments increases the impression that the 

 conditions of formation of these artificial structures are of the 

 same fundamental nature as those of actual cell-structures. 



Other metals which form insoluble ferricyanides may give rise 

 in the above solution to filaments of a similar type, differing 

 characteristically in their structure, appearance, and rate of 

 formation from those of iron ferricyanide. Small pieces of zinc 



