28 The Beginnings of Multicellular Organization 



the supply of bacteria is exhausted, the cells stop growing and enter the 

 aggregation stage. The myxamoebae stream radially toward central col- 

 lecting points, and, as they reach the center, a conical mound of cells is 

 built up. When all the cells have aggregated, each conical mound falls 

 over on its side and is transformed into a worm-like slug up to a few 

 millimeters in length. The slug migrates over the substratum and finally 

 comes to rest. It then proceeds to construct a fruiting body with a round 

 mass of spores at the top, a stalk enclosed in a cellulose sheath below, and 

 a basal disc at the bottom. These constitute three clearly different cell 

 groups. Ultimately the spores are cast oflF to repeat the cycle while the 

 stalk cells and basal disc cells desiccate and die. Figure 13 shows photo- 

 graphs of an aggregation sequence, migrating slugs, and the construction 

 of a fruiting body. 



The cellular slime molds display the same attributes that more com- 

 plicated developmental systems, i.e., embryos, do. These are categorized 

 as cellular differentiation, cell interactions during development, and 

 regulation. 



CELLULAR DIFFERENTIATION 



As we have mentioned, the fruiting body consists of three distinct 

 cell groups: spores, stalk cells, and basal disc cells. These have radically 

 different sizes, shapes, constitutions, and functions— as do the various 

 tissue cells of a frog embryo. We can ask: (a) whether cells normally 

 destined to become spores can develop instead into stalk cells? and (b) if 

 a stalk cell or basal disc cell were removed from the fruiting body before 

 desiccation and death and permitted to grow and reproduce, would its 

 progeny develop normally, i.e., could they aggregate and build a fruit 

 containing all three tissues? 



The fate of a cell (i.e., whether it will become a spore or stalk cell 

 or basal disc cell) appears to depend on the order in which it enters the 



Fig. 13. Slime mold morphogenesis. Leff, an ag- 

 gregation sequence: top right, migrating slugs 

 (from J. T. Bonner, The Cellular Slime Molds. 

 Princeton: Princeton University Press, 1959, Plate 

 II); bottom right, the stages of fruiting body 

 construction — the photographs were taken ap- 

 promixately I '/2 hours apart (also from Bonner, 

 Plate III). 



