INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 435 



critical value, when a new process starts. This is aggregation. The amoebae 

 move together into streams which converge on a certain region, the 

 aggregation centre, at which they become heaped together to form a 

 sausage-shaped cell mass (Fig. 20.8). This mass then starts moving as a 

 whole, creeping over the surface at a speed comparable to that of the 

 individual amoebae (approximately 2 mm. per hour). After a time the 



FiGxjRE 20.8 



Four stages in the aggregation o£ Dictyostelium from the amoeboid phase to 

 a compact mass. (From Bonner 1952.) 



creeping movement stops, and the heap gradually rears itself up into a 

 peg-like structure, with a lump of cells (which become spores) at the 

 top of a thin stalk which stands on a small expanded base (Fig. 20.9). 

 Now it has been shown (particularly by Bonner 1947, 1952) that the first 

 phase of these morphogenetic movements, namely the aggregation, 

 occurs under the influence of a substance, known as acrasin, which is 

 given off by the amoebae. Each amoeba gives off this substance, and at 

 the same time tends to move along any gradient of acrasin that may be 

 present. If sufficient amoebae happen to close together they form a 



