Polarity 143 



whole cytoplasmic body of the organism, whether this is cut up into cells 

 or not. 



Polar patterns are not confined to organisms that have developed in the 

 ordinary way by growth from a reproductive unit such as a spore or egg 

 but are found in what are essentially organic communities. Thus in cer- 

 tain slime molds such as Dictyostelium (p. 223) the vegetative individual 

 is a tiny myxamoeba. At the end of vegetative growth some thousands of 



Fig. 6-15. Polarity in Dictyostelium. If an apical piece of a pseudoplasmodium is placed 

 in close contact with the apical end of another, coalescence takes place. If it is placed 

 next the basal end, either of the same plasmodium or of another, there is no coa- 

 lescence, and the terminal piece moves off independently. (From K. B. Raper.) 



these become aggregated into a pseudoplasmodium where each retains its 

 individuality. This colonial structure shows a polar organization, for the 

 terminal portion of it can be grafted to the decapitated apex of another 

 pseudoplasmodium, though not to the base (Fig. 6-15). The tip is evi- 

 dently the dominant region, for if grafted to the side of a pseudoplas- 

 modium it will withdraw from it a group of individuals and start out as a 

 new unit. The sorocarp that ultimately develops has a vertical polar axis 

 and, in some species, lateral axes as well. Polarity in organisms like these 



