The Beginnings of Multicellular Organization 27 



uality of these cells is clearly submerged within the colony and we can 

 begin to talk of the colony as the individual. Thus botanists are accus- 

 tomed to call a colony of a higher mold a "plant" as if it were— as indeed 

 it is— a single functional entity. 



Multicellularity requires that nature must solve many problems re- 

 lating first to the formation of the cell community and second to its proper 

 functioning, problems that do not arise in isolated cells. Organisms that 

 display primitive multicellular organization are of great interest to biolo- 

 gists because they illustrate nature's first efforts to develop an individual 

 beyond the single cell. Furthermore, by learning the genetic and bio- 

 chemical mechanisms of primitive morphogenesis, we hope to uncover 

 principles that are general to all multicellular organisms, including higher 

 plants and animals. 



The Cellular Slime Molds 



Figure 12 summarizes the life cycle of one species of cellular slime 

 mold called Dictyostelium discoideum. The cycle is divided into four 

 stages: growth, aggregation, migration, and fruiting-body construction. 

 It starts with the germination of spores to yield amoeboid cells termed 

 myxamoebae. The myxamoebae live in the soil or on the agar surface of 

 a Petri dish, feed upon bacteria, and reproduce by binary fission. When 



Fig. 12. The life cycle of 

 Dicfyosfelium discoideum. 



