curred (Fig. 9). Exactly what the mechanism 

 is that caused the differentiation, of course, is 

 a problem. 



CHALKLEY: Is this a sharp or slow tran- 

 sition? 



GREGG: If you look at enough of these, you 

 can see small areas in the late aggregate that 

 have begun to differentiate. Presumably, be- 

 tween the time they are standing up like this 

 and the time they flop over they differentiate 

 most of their prestalk cells. Now, it's impossi- 

 ble to say in this preparation how long this 

 particular slug has been migrating. 



KAHN: It might be worth pointing out that 

 this process of tipping over takes no more than 

 a few minutes. 



GREGG: Yes. So differentiation may begin 

 just prior to flopping and is completed in a 

 relatively short time. 



B. WRIGHT: Do you think this difference 

 in staining intensity could be a difference in 

 permeability to the stain? 



GREGG: I don't think so because these are 

 histological sections, of course, and I wouldn't 

 think that cell permeability is involved here. 



B. WRIGHT: Perhaps the spore cells have 

 a more resistant coating. 



GREGG: Since these are no longer whole 

 cells, having been sectioned, I don't think a 

 permeability factor could be involved. 



LOVETT: You showed the two kinds of 

 amoebae in the aggregating stream of D. muco- 

 roides. Are they the same in this respect? 



GREGG: Yes, they' re present both in D. dis- 

 coideum and Z). mucoroides. 



LOVETT: Can't you see them at all when 

 it's still erect? 



GREGG: You cannot distinguish two types 

 of cells once the aggregate has formed. You can 

 detect them only when you look at the separate 

 amoebae taken from a late interphase or an 

 aggregate. 



LOVETT: If they're just lost in the mass, 

 I wonder if they could creep up. 



GREGG: Yes, and that is just the reason 

 you cannot exclude sorting out. However, it's 

 just amazing that by the late aggregate in 

 D. discoideum you see very little evidence of 

 prestalk cell differentiation. I would think that 

 if sorting out was going to occur, it would occur 

 as part of this process in elongating the cell 

 mass. It's surprising, if it is one of the proc- 

 esses of slug formation, that you do not see 

 more prestalk cell differentiation at this time. 



KAHN: I want to ask another question about 

 the staining. Are the classes of light and dark 



Plate I. Figures 2 through 13 



Fig. 2. D. mucoroides aggregating myxamoebae exhibit- 

 ing different degrees of staining with homologous fluores- 

 cent antiserum (HFAS). 



Fig. 3. D. mucoroides early aggregate exhibiting uniform 

 staining throughout the cell mass (HFAS). 



Fig. 4. D. mucoroides late aggregate exhibiting the ini- 

 tial differentiation of the anterior prestalk cells as indi- 

 cated by the decreased cytoplasmic staining. Prespore 

 antigen synthesis has started In certain cells of the 

 posterior prespore area. Staining observed on all cell 

 surfaces (HFAS). 



Fig. 5. D. mucoroides slug exposed to homologous fluo- 

 rescent normal serum. Staining Is completely negative. 



Fig. 6. D. mucoroides slug exhibiting Intense staining 

 with homologous fluorescent antiserum (HFAS) in the pre- 

 spore area but lacking cytoplasmic staining In the pre- 

 stalk and stalk cells. All surfaces show staining. 



Fig. 7. D. discoideum early aggregate exhibiting uniform 

 staining with homologous fluorescent antiserum (HFAS). 



Fig. 8. D. discoideum late aggregate exhibiting uniform 

 staining throughout the cell mass (HFAS). 



Fig. 9. D. discoideum slug exhibiting Intense staining in 

 the prespore area but lacking cytoplasmic staining In the 

 prestalk area. All cell surfaces show staining (HFAS). 



Fig. 10. D. discoideum preculm exhibiting an Intense 

 staining in the prespore area but negligible cytoplasmic 

 staining in the prestalk, stalk and basal disc cells. All cell 

 surfaces are stained (HFAS). 



Fig. 11. D. discoideum culminating exhibiting a de- 

 creased Intensity of cytoplasmic staining In the prespore 

 area as compared to the preculm. Prestalk and stalk cells 

 stained on cell surfaces (HFAS). 



Fig. 12. Group of D. discoideum spores exhibiting a de- 

 creased degree of cytoplasmic staining as compared to 

 preculm. Cell surfaces retain their staining capacity 

 (HFAS). 



Fig. 13. MV slug exhibiting the lack of well defined pre- 

 stalk and prespore regions as noted in D. mucoroides slugs 

 (D. mucoroides FAS). 



(Figs. 2, 3, 7-12 from Gregg, Devel. Biol. 12, 377, 1965; 

 reproduced with permission of Academic Press.) 



