CELLULAR DIFFERENTIATION IN THE SLIME MOLD 233 



increased I-cell frequency stems partly from the increased stability of 

 the phenotype in the immediate progency of the anomalous I-cells but 

 mostly from a ten-fold increase in the rate of appearance of I-cells 

 during the growth of the anomalous R-cells. It is both interesting and 

 comforting to note that the alteration of the I-cell frequency is accom- 

 panied by a change in the aggregative performance. 



In summary, approximately one of 20,000 amobae is an anomalous 

 I-cell, and its descendants possess potentialities for ploidal transforma- 

 tion and aggregative performance which differ from those displayed 

 by the descendants of all the other I-cells and R-cells. The proportion 

 of anomalous cells is so low that it contributes negligibly to the over-all 

 phenotypic make-up of the population, but it does represent a reservoir 

 of genetic variation ( particularly for the genesis of diploid cells ) which 

 could be of considerable importance. 



Ross has recently reported the existence of stable haploid and 

 diploid strains of D. discoideum, as evidenced by direct chromosome 

 counts ( 1959 ) . The anomalous I-cell clones are seen to constitute a 

 third kind of population in which both haplophase and diplophase are 

 represented in appreciable numbers arid in which transformation be- 

 tween the two occurs at a high rate. It may thereby act as a bridge 

 between the two stable varieties, permitting the selection of either from 

 either. 



Modes of cell division. The fission cycles of 35 or 40 I-cells and a 

 few hundred R-cells have been recorded by time-lapse cinematography. 

 The films divulged a most remarkable difference between the two, and 

 this is illustrated by the frames reproduced in Figure 6. The recorded 

 R-cell divisions were normal binary fissions without any exceptions, and 

 about 50 per cent of the I-cell divisions were also of this nature, but 

 the remainder were trinary or quaternary fissions to yield three or four 

 daughters! The binary I-cell divisions yielded daughters one or both of 

 which retained the parental phenotype. The multiple fissions resulted 

 in the appearance of R-cell progeny. 



Sex? 



For the greater part of their existence as objects of biological in- 

 vestigation, the cellular slime molds have been thought to be celibate 

 organisms. An early report of syngamy in D. mucoroides (Skupienski, 

 1918) was dismissed, largely on the basis of associated observational 

 errors. In recent years Wilson (1952, 1953) published cytological data 

 purporting to demonstrate the existence of syngamy and meiosis in D. 

 discoideum, and he proposed a mandatory sexual addendum to its life 

 cycle. Arguments raised by others (Sussman, 1955, 1956) and subse- 

 quent observations by Wilson and Ross (1957) were followed by a 



