CELLULAR DIFFERENTIATION IN THE SLIME MOLD 



231 



grows replicate microcultures, each one starting from a very small 

 inoculum of R-cells. After growth the replicates are scored for the 

 presence of I-cells. From the proportion of those without I-cells and 

 the mean number of cells per culture, one can calculate the rate of 

 appearance. Seven experiments of this kind yielded a mean value of 

 2.2 X 10"^ I-cell appearances per R-cell division cycle. In other words, 

 one I-cell appears during the time it takes 1,600 R-cells to grow and 

 divide into 3,200. 



The phenotijpic composition of clones derived from I-cells. The 

 progeny of I-cells, cultivated in clonal isolation, have been followed 

 during the first five to ten generations of exponential growth by serial 

 observations at frequent intervals or by time-lapse cinematography and 

 once again after many generations and several subcultures. The re- 

 sults of this survey are: 



1. The I-cell phenotype was lost by cell division only. About 10 to 

 15 per cent of the I-cells yielded no issue at all, and others experienced 

 a lag of many hours before division. Such cells moved normally, fed 



1:510 



20r 



10 - 



CLONE 1-5 



1:2040 



10 



20,000 

 CELLS 



200 



2130 



400 600 



CLONE R-3 



10 



200 



400 



20,000 

 CELLS 



600 



CLONE I- 2 A 



1:550 ^-O IO,OOOCELLS 

 5.000 CELLS 



100 200 



1 : 2080 



300 

 NC-4 STOCK 



20,000 

 CELLS 



200 



400 



600 



CELL DENSITY IN CELLS/min 



_2 



Figure 5. Aggregative performances of the working stock, NC-4, and clones derived 

 from I-cells and an R-cell. The data for clone 1-5 were obtained in two separate experi- 

 ments, and the data for NC-4 in five experiments, and they illustrate the reliability of the 

 assay. (Sussman ef a^, 1960.) "' 



