250 RALPH S. LILLIE AND EARL N. JOHNSTON. 



of growth deserves to be studied as a necessary preliminary to 

 the theoretical study of form, and, mathematically speaking, 

 organic form itself appears to us as a function of time." 1 The 

 rate of formation of all these structures, which depends on the 

 rate of local electrolysis, is always most rapid at first and slows 

 down as the reaction continues. Both internal and external 

 conditions enter to cause the retardation. The frictional resis- 

 tance to the outflow of the solution from the tubular filament and 

 the electrical resistance of the circuit are proportional to the 

 length of the filament. As these increase the rate at which the 

 precipitate is formed decreases; the rate of growth is consequently 

 lowered. There is an analogy here to the formation of living 

 organic structures, where growth is always more vigorous at 

 first and slows down as development continues. Child says, in 

 his chapter on rate of metabolism, "everywhere the rate of 

 growth is high in the young organism or in the young cells and 

 tissues of the organism, and decreases as development proceeds 

 and the rate of metabolism falls. With adequate nutrition and 

 external conditions which permit growth the rate of growth ap- 

 pears to be in a general way dependent on the rate of metabo- 

 lism"; 2 and he points out that the accumulation of structure 

 formed in metabolism may itself impede the metabolic process 

 by which more structure is formed. The case of precipitation- 

 structures offers an interesting analogy. 



The influence of external conditions on rate and character of 

 growth will be discussed below. 



Branching can be produced in large filaments by perforating 

 their walls with a finely pointed needle or glass rod. If this is 

 done while the filament is growing, the branches resemble the 

 main filament in almost every particular (Fig. 7). As many as 

 seven branches were produced in this way, each new branch 

 being formed from the preceding one, and sometimes three were 

 growing at the same time. Puncturing the membrane after a 

 filament has ceased to grow forms small side branches which 

 themselves almost immediately cease to grow (Fig. 8). 



Cross-striation is frequent in Fe filaments as well as in filaments 



1 Thompson, loc. cit,, p. 51. 



- C. M. Child, "Senescence and Rejuvenescence," Chapter XL, the Rate of 

 Growth, p. 276. 



