222 CELLS, TISSUES, AND ORGANISMS 



(Gregg, 1956, I960; Sonneborn et al., 1960) and by dramatic changes 

 in enzyme activities ( Wright and Anderson, 1958, 1959 ) . 



Recently Drs. Raquel Sussman, H. L. Ennis, and I have been in- 

 volved in the study of an example of phenotypic heterogeneity among 

 exponentially growing myxamoebae— that is to say, long before the 

 onset of the morphogenetic sequence. When strain NC-4, a haploid 

 representative of D. discoidcum, is grown in association with Aero- 

 bacter aerogenes or Escherichia coli on a variety of media, one can 

 recognize two cell types on the bases of size, flatness, vacuolation, and 

 motility. They have been termed I-cells and R-cells and have been 

 found to be present throughout the log phase in a steady-state ratio of 

 about 1:2000, respectively (Ennis and Sussman, 1958a; Sussman and 

 Ennis, 1959; Sussman et al., 1960). 



Although the definition of the two cell types rests only upon 

 morphological grounds and is independent of other attributes, the dif- 

 ferentiation extends to their morphogenetic capacities as well. Specifi- 

 cally, a high proportion of I-cells can initiate the formation of aggre- 

 gates among their neighbors, whereas the R-cells, under identical test 

 conditions, cannot. Finally, the I-cells demonstrate remarkable genetic 

 properties, as yet only partly understood, which point to a sexual or 

 parasexual function. 



In what follows I shall attempt to summarize the information 

 bearing upon the aforementioned properties, but with the qualification 

 that the information is in some respects fragmentary and in all respects 

 incomplete. 



The morphological distinctions 



Typical I-cells and R-cells are shown in Figure 1, and the gamut of 

 morphological diflFerences thus far observed is summarized in Table I. 

 The initial distinction is made by surveying myxamoebae at lOOX mag- 

 nification. Most of the cells are small ( about 10 to 20 microns in mean 

 diameter) and are hemispherical in contour, but occasionally one's at- 

 tention is caught by a large (about 30 to 50 microns), very flat, and 

 highly vacuolated myxamoeba. Examination under 400X magnification 

 confirms the identification of the I-cell by revealing its heavy granula- 

 tion and extreme and characteristic pseudopodial activity. Although 

 present in small proportion, these cells can be recognized and enumer- 

 ated by independent observers with high precision. 



As Table I indicates, other differences exist. I-cells frequently 

 engulf their neighbors — a practice reminiscent of the plasmodia of true 

 slime molds. An appreciable number are multinucleate, as many as five 

 nuclei having been detected in a single cell, whereas R-cells are uni- 

 nucleate ( or binucleate immediately prior to cell division ) . When uni- 



