228 CELLS, TISSUES, AND ORGANISMS 



able distances ( Siissman and Ennis, 1959), and that the I-cell need only 

 be in contact with its neighbors for a remarkably short time in order to 

 exert its effect and make its subsequent removal irrelevant to the subse- 

 quent course of aggregation (Ennis and Sussman, 1958a). These and 

 other experiments, as yet at a preliminary stage and unpublished, all 

 point to the svipposition that initiation involves the deposition of a 

 diffusible substance which, taken up by the responder cells, induces 

 the onset of aggregation. Though this is by no means certain and must 

 be considered with great caution at present, it does seem the most likely 

 mechanism. The different initiative capacities of I-cells and R-cells could 

 then be ascribed to differential rates of production of the "initiator sub- 

 stance." Thus, at the cessation of growth and the start of the pre- 

 aggregative period, the I-cells would produce the material at a high 

 rate and the R-cells at a low or negligible one. The rates of production 

 by all the cells would increase with time. If, then, a local discontinuity 

 in the concentration of the substance were necessary in order to signal 

 the start of aggregation, the amount deposited by an I-cell would be 

 discernible over the general noise level contributed by the R-cells.^ In 

 contrast, the production of initiative material by no single R-cell would 

 be sufficient to be distinguishable above the noise level of its develop- 

 mental contemporaries, but it might be if that R-cell were in the 

 presence of its developmental juniors. 



At the moment these considerations are extremely speculative and 

 can do nothing more than suggest what investigative approaches might 

 be fruitful in the immediate future. It seems to us that they point 

 compellingly to the need for a biochemical definition of the initiative 

 act. Several possible bioassay systems are available for the detection of 

 the hypothetical initiator substance and could be employed in its puri- 

 fication and identification. 



Genetic properties 



The phenotijpic composition of clones derived from R-cells. Clones 

 derived from R-cells have been examined with respect to four criteria. 

 They are: general cell size distribution, I-cell frequency, ploidy, and 

 aggregative performance. The latter term refers to how many aggrega- 

 tive centers given numbers of cells can form under certain specified 

 conditions, and at what population densities they can do so. 



Thus far, all those examined have appeared to be identical with 

 one another and with the carried stock of D. discoideum NC-4 (hap- 

 loid), which is itself maintained in a homogeneous state by periodic 

 re-isolation from single, randomly selected clones. Figure 3 illustrates 

 the size distribution observed among vegetative myxamoebae, and 

 Figure 4 shows corresponding data for spores taken from the fruits. 



