CELLULAR DIFFERENTLVTION IN THE SLIME MOLD 225 



basis alone, the distinction between the two classes would seem to be 

 justified. 



Morphogenetic capacities of I-cells and R-cells 



Figure 2 shows a series of time-lapse photomicrographs of a typical 

 aggregation carried out by cells on washed agar. The arrow in the first 

 photograph points to the centrally located I-cell, initially approached 

 by only one of its nearest neighbors but later by others until a clump 

 has formed about it. Meanwhile peripheral clumps have appeared— a 

 situation not encountered during the pre-aggregative period. Suddenly 

 myxamoebae within and outside the clumps elongate, ramified streams 

 take shape, and the aggregative pattern now emerges with clarity. The 

 center of the aggregation is seen to coincide with the last recognizable 

 position of the I-cell. About 50 per cent of the aggregates studied have 

 shown an identical course of events. The remaining 50 per cent were 

 quite similar, save that the center formed very near to but not at the 

 I-cell. 



The possibility that I-cells can initiate centers of aggregation, sug- 

 gested by these photographs, could be confirmed in a more direct man- 

 ner. \Mien replicate samples of 250 myxamoebae were incubated at 

 high density on agar, only about 10 per cent of them aggregated. The 

 addition of an I-cell to each sample increased the incidence to about 

 70 per cent ( 80 per cent in the best experiment ) , whereas the addition 

 of R-cells left the background incidence of aggregates unaffected. The 

 data supported two general conclusions : 



1. That a single cell can initiate the formation of an aggregate— a 

 conclusion strongly indicated by prior results (Sussman and Noel, 

 1952; Sussman, 1952, 1956 ) but established directly for the first time in 

 these experiments. 



2. That under identical conditions of assay, I-cells possess a high 

 level of initiative capacity and R-cells do not. 



Given this degree of morphogenetic heterogeneity, one can ask if 

 the presence of I-cells does not account for all the aggregates that a 

 population of myxamoebae can produce. Under some conditions, at 

 least, the I-cells do appear to account for all the observed aggregations.. 

 Thus, when NC-4 myxamoebae, harvested from the stationary phase 

 and washed free of bacteria, are dispensed on washed agar, the number 

 of aggregative centers ( one per 2,200 cells ) is in close agreement with 

 the number of I-cells (one per 1,940 cells). The incidence of aggregates 

 among small population samples can be predicted with extremely high 

 accuracy on the basis of whether an I-cell is present or not. Removal of 

 I-cells, if done early enough, appears to stop aggregation (Ennis and 

 Sussman, 1958a). 



