spaced 



10 



random 



25 50 



Nearest neighbor distance 



Fig. 3. 



The relation between the density of aggregates and nearest 

 neighbor distance. The curve depicts the relationship ex- 

 pected if the spatial distribution of centers is at random. 

 Deviations to the right of the curve indicate a "spaced" 

 distribution; deviations to the left, a "clustered "distribu- 

 tion. 



drops on buffered non-nutrient agar. Counts 

 of the number of aggregates were made in all 

 cases after 24-26 hours of incubation. In some 

 cases, counts were also made at hourly inter- 

 vals to determine the rate of center formation. 

 Nearest neighbor distances were obtained with 

 an ocular micrometer. 



When counts and measurements were made 

 on a number of aggregating populations, it was 

 found that all three types of distribution oc- 

 curred. Random distributions were the most 

 frequent, followed by spaced and then clustered. 

 Since spaced distributions occur in the presence 

 of charcoal (an agent that should remove the 

 spacing substance), it suggests, but does not 

 prove, that such spaced distributions are not the 

 result of a gaseous spacing substance. In- 

 terestingly, spaced distributions were most 

 often observed in low center density situations 

 while clustered distribution were associated 

 with high density. 



The correlation between center density and 

 distribution led to a consideration of those 



c 

 o 



o 



E 



c 

 o 



D 



o 



10 100 



Log density of centers 



Fig. 4. 



The relationship between the rate of center formation 

 (the number of centers appearingper unit of time) and the 

 final density of aggregation centers on the substrate 

 (surface). Note that the faster the rate of center forma- 

 tion, the higher the final density. 



factors or phenomena that determine center 

 density. One, apparently fundamental, relation- 

 ship is illustrated in Fig. 4. Note that the 

 faster the rate of center formation, the higher 

 the density of centers. 



The next step, then, was to ascertain those 

 factors which play a role in determining the 

 rate of center formation. The influence of a 

 number of such factors are shown in the graphs 

 in Fig. 5. 



Figure 5A illustrates the rate of center 

 formation as a function of stage in the growth 

 cycle. Note that stationary phase cells begin to 

 aggregate the moment they are placed on the 

 substrate, while logarithmic phase cells do so 

 only after a lag of two hours. Furthermore, once 

 aggregation begins, log phase cells proceed 

 at a much slower rate than do stationary phase 

 cells. 



Amoebae which are incubated in the light 

 and in the presence of charcoal or mineral 

 oil, aggregate much faster than comparable 

 amoebae incubated in the dark and in the ab- 

 sence of these two factors (Fig. 5B). Charcoal 

 and mineral oil are believed to remove a center 

 suppressing factor present in the environment 

 while light is believed to mitigate the effect of 

 this factor (7). 



126 



