Differentiation 225 



two regions. If the apical and the basal halves of the pseudoplasmodium 

 are experimentally separated, each will form a sorocarp, but the one from 

 the apical half produces the larger spores. Despite these evidences of 

 the beginning of differentiation, cells in one group can be changed to 

 resemble those of the other, and every cell apparently is totipotent. If a 

 few cells are removed from the mass they are no longer subject to its 

 organizing control, and if food is present, they will become vegetative 

 cells again and proceed to multiply. 



That the pseudoplasmodial axis is polarized is shown not only by the 

 difference in structure of its two ends but by their behavior. K. Raper 

 (1940Z?) performed a series of grafting experiments between plasmodia 

 that could be distinguished by their color, one group having fed on red 

 bacteria. The apex, if cut off and placed at the rear of another plasmo- 



DICTYOSTELIUM DISCOIDtUM 



Fig. 8-24. Migration of the pseudoplasmodium and formation of the sorus in Dictyo- 

 stelium discoideum. (From ]. T. Bonner.) 



dium, will not fuse with this one but will start off by itself. It fuses 

 with the apical end of a decapitated plasmodium (Fig. 6-15). If an 

 apex is cut off and placed next the side of an intact plasmodial mass it 

 will attach itself there and finally draw off a considerable mass of cells 

 and establish a separate plasmodium. The tip of the mass sometimes 

 splits, and in this case two are formed. If two happen to come together, 

 they may fuse into a single one of double size. 



Morphogenetically the most significant part of the life history is the 

 formation of the stalked fruiting body, or sorocarp. After a few hours of 

 migration, or when a pseudoplasmodium reaches drier surroundings, it 

 stops moving and attaches itself firmly to the substratum by a disk of dif- 

 ferentiated cells (Fig. 8-24). The cells of the apical region, from which 

 the stalk is to be formed, become large and vacuolate and each is en- 

 closed in a cellulose sheath. As this is happening, they are pushed down 

 into the pseudoplasmodium by other prestalk cells climbing up around 



