224 



CELLS, TISSUES, AND ORGANISMS 



TABLE I 



Differences between I-cells and R-cells 



Criterion 



I-Cells 



R-Cells 



Morphology 



Motility 



Inclusions 



Nuclear 

 constitution 



Modes of cell 

 division 



Morphogenetic 

 Potential 



Genetic 

 stability 



Large, flat, heavily granulated Small, hemispherical, 

 and vacuolated lightly granulated and 



vacuolated 



Very active, with explosive 

 pseudopodia and lobopodia 



Engulfed cells very common 



Much less active 



Engulfed cells very rare 



One large or two to five small 

 nuclei 



One small nucleus or two 

 small nuclei, immedi- 

 ately before cell division 



Ternary or quaternary fissions Only binary fissions were 

 to yield three or four obsei-ved 



daughters, observed very 

 frequently 



Under identical test conditions, I-cells display a high ca- 

 pacity to initiate centers of aggregation, whereas R-cells 

 display a low or negligible one. 



All of the clones derived from R-cells and about 90 per 

 cent of those from I-cells attain identical phenotypic com- 

 positions with respect to size distribution, ploidy, I-cell 

 frequency, and aggregative perfoiTnance. But 10 per cent 

 of the clones derived from I-cells are heritably anomalous 

 with respect to these criteria. 



nucleate, the I-cell nucleus is considerably larger than that of R-cells or 

 multinucleate I-cells. R-cell metaphase figures fixed with osmic acid 

 and stained with aceto-orcein by the method of Ross ( 1959 ) invariably 

 display the haploid number (7) of chromosomes. The ploidy of the 

 I-cells is still unknown, but the technical problems attendant upon iso- 

 lating an I-cell, fixing it at metaphase, and staining it can probably be 

 solved so as to yield an unequivocal answer. 



At present it is unclear whether the I-cells and R-cells are discrete 

 entities or merely represent extremes in a continuum of cell sizes and 

 correlated properties. A crucial distinction between these alternatives 

 ought to be provided by the determination of the size distribution of a 

 few hundred thousand myxamoebae, but this is not technically feasible 

 now. In any event, the setting aside of the I-cells as objects of special 

 study, whether chosen on arbitrary grounds or not, has shown them to 

 dijffer from the remainder of the population in many interesting and 

 apparently important ways which demand further elucidation. On this 



