230 



CELLS, TISSUES, AND ORGANISMS 



40 r 



20 



NC-4 



' I I I I I •— a— ^c2q 



9.1 12.5^1+1.4 15.8 



40 



20 



10 14.5 ;j± 2.2 



22.6 



12.5 I68jj±2.4 



25.1 



Figure 4. Size distributions of spores. Abscissae: the sums of major and minor 

 diameters. Ordinates: numbers of spores. The mean sums and standard devi- 

 ations are given for each strain. The histogram at the top was redrawn from 

 data published by Bonner and Frascella (1953) for NC-4. 



and stained with aceto-orcein ( Ross, 1959 ) . Only haploid chromosome 

 sets were detected in the R-cell clones and the carried stock ( as against 

 only diploid sets in strain RA ) . The aggregative performances of a typi- 

 cal R-cell clone ( R-3 ) and of the carried stock are shown in Figure 5. In 

 both the response of center-formation to population density was the 

 same, with the optimal density falling at about 200 cells per square 

 millimeter. The ratio of centers formed at the optimal density to cells 

 present was about 1:2100 in both instances. 



The foregoing results indicate that I-cells arise from and during 

 the growth of R-cells and attain a steady-state ratio of 1:2000 under the 

 particular cultural conditions employed. Other and related properties 

 (cell size, ploidy, aggregative performance) also follow an invariant 

 pattern under these conditions. 



The rate of appearance of the I-cells was crudely estimated by the 

 null method of Luria and Delbruck (1943). For this purpose, one 



