30 The Beginnings of Multicellular Organization 



aggregate ( Fig. 14 ) . Cells that enter the aggregate first become the lead- 

 ing element of the migrating slug and ultimately the lower stalk of the 

 fruit. Later arrivals take up progressively more posterior positions in the 

 slug and become upper stalk cells and spores. Those entering last comprise 

 the tail end of the slug and ultimately the basal disc of the fruit. One can 

 tentatively imagine that the position of a cell in the aggregate and in the 



Fig. 14. The fate of fhe cells depends on the order in which they enter 

 the fruit. 



slug subjects it to a specific set of environmental conditions and of chem- 

 ical signals imposed by its neighbors that forces it to become a spore or 

 stalk cell or basal disc cell. These differences are reversible. Slugs have 

 been cut into head and tail segments. Both could construct fruits sepa- 

 rately. Although not completely normal, both contained all three types of 

 cells even though the head segment would ordinarily have given rise to 

 stalk only, while the tail segment would have yielded spores and basal 

 disc cells but not stalk. In addition, stalk cells have been separated from 

 the fruiting body before they desiccated and died and have been allowed 

 to feed on bacteria and reproduce. The progeny of each such stalk cell 

 were themselves capable of aggregating and constructing normal fruits 

 with viable spores. 



Another example of cellular diflPerentiation is seen in the migrating 

 slug. If the front end of the slug ( about 10 per cent of the cells ) is sepa- 

 rated from the rear end, it continues to migrate, while the rear end stops 

 in its tracks. If the front end is replaced, the two parts fuse, and migration 

 of the whole slug proceeds once again in normal fashion. 



These and other experiments demonstrated that two cell types, 

 "leaders" and "followers," exist and that the slug is an organized entity. 

 The biochemical mechanisms by which the leaders guide the migration 

 of the followers and the manner in which cells become leaders and fol- 

 lowers are still unknown. 



