226 The Phenomena of Morphogenesis 



them which in turn become stalk cells (Fig. 8-25). As Bonner describes 

 it, "The process is the reverse of a fountain; the cells pour up the outside 

 to become trapped and solidified in the central core which is the stalk. 

 In so doing the whole structure rises into the air until all the prestalk 

 cells have been used up." This description applies to the species most 

 commonly studied, Dictyostelium discoideum. In D. mucoroides and D. 

 purpureum, however, the stalk begins to be formed during the brief 

 migration of the pseudoplasmodium. 



About 10 per cent of the myxamoebae take part in the formation of 

 disk and stalk. The others, still moving freely over one another, follow 

 the growing tip of the stalk upward in a body and (in Dictyostelium) 



O Undrfferer*oled 

 © Supportive 

 Spores 



n 



Fig. 8-25. Dictyostelium discoideum. Diagram of sorocarp formation. A, B, migrating 

 pseudoplasmodium. ( From J. T. Bonner. ) 



form a spherical mass of cells, the sorus, at its summit. Here each myxa- 

 moeba rounds up to make a dry spore, and these spores are later carried 

 away by air currents, each now capable of developing into a myxamoeba. 

 Wilson presents evidence that some mitotic division occurs before spore 

 formation. 



In the genus Polysphondylium there is not only an apical sorus but 

 several whorls of lateral stalks along the main one, each terminating in 

 a smaller sorus, so that the sorocarp becomes a structure of considerable 

 complexity. 



These plants, though so unlike higher ones, have constant generic and 

 specific differences ( Fig. 8-26 ) . If myxamoebae belonging to two species 

 grow intermingled in the same culture or if pseudoplasmodia of two 

 species are crushed and experimentally mixed, the cells in time sort them- 



