cells, if any at all. You see occasional mitotic 

 figures but apparently there is no significant 

 increase in cell number. 



FERGUS: Most of this has occurred prior 

 to the formation of the slug? 



GREGG: Yes. As a matter of fact, division 

 apparently does not occur after aggregation. 

 Furthermore they utilize only endogenous food- 

 stuff during morphogenesis. 



FERGUS: You were working here with no 

 external source of food? 



GREGG: Yes. They feed upon bacteria dur- 

 ing vegetative amoebae stage; and once they 

 aggregate, they can carry out this whole devel- 

 opmental process in the complete absence of 

 foodstuffs. 



PERSON: Is this a buffered medium? 



GREGG: This is on agar; they're buffered 

 at about 6.2. Complete morphogenesis occurs 

 on this medium. 



GROSS: This certainly ought to dispel one 

 of the pet ideas of a number of embryologists 

 that is still quoted very widely: namely, that 

 differentiation and dedifferentiation are proc- 

 esses that are intimately linked with cell divi- 

 sion. Dedifferentiation itself is not demonstrated. 



GREGG: This appears to be a form of 

 dedifferentiation. 



DEERING: You have type a (prespore) 

 changing to type b (prestalk) or type b changing 

 to type a, either way? 



GREGG: Yes, and this occurs in the ab- 

 sence of an increase in the mass of cells. 



KAHN: I think that this is a very interest- 

 ing point. I must confess that I've always felt 

 that cell differentiation (morphogenesis) in these 

 organisms was independent of cell division, but 

 I'm beginning to think, in terms of this experi- 

 ment, that this point should be tested. After all, 

 these amoebae do have a fair amount of endo- 

 genous reserve. For example, if spores are 

 placed in a suitable environment they will ger- 

 minate and may complete the life cycle a second 

 time in the absence of exogenous nutrient. 



GROSS: Are you implying that there is cell 

 replication? 



KAHN: I'm implying that it's possible. 



GROSS: However, in order to have some- 

 thing that approximates the old hypothesis that 

 the decision is made at mitosis, you'd have to, 

 at least, double the number of cells. I should 

 think that could easily be seen. 



GREGG: I don't believe that cell division 

 is necessary, because you can cause fruiting 

 body formation from small quantities of cells. 

 As a matter of fact, fruiting bodies have been 



obtained from as low as 7 cells, 



GROSS: Is that an adult fruiting body? 



GREGG: Yes. Obviously cell division isn't 

 necessary here although it's true no one has 

 examined a larger isolated anterior tip. 



KAHN: I think there's apoint worth stress- 

 ing about regulation (developmental) in cellvilar 

 slime mold development. For example, Bonner 

 has shown that normal development can occur 

 in aggregates containing fewer than 100 cells as 

 well as in aggregates containing many thousands 

 of cells. In the slime mold, Acytostelium, even 

 a single amoeba may show developmental regu- 

 lation. In this case the cell gives rise to a struc- 

 ture composed of a single spore perched on an 

 acellular stalk. 



GROSS: At any rate, I think it's helpful to 

 the state of the problem so as to have these 

 things discussed more widely than they are. 

 Most people don't know about this particular 

 point. It's such a clear case of a switch in the 

 choice that the cell makes about what it's going 

 to do, a switch that can be produced externally 

 without any massive cell replication. 



GREGG: It's one of the most striking things 

 about cellular slime molds. 



GROSS: If it's true that these cells are 

 really not replicating, then all of this may hap- 

 pen during interphase. This immediately rules 

 out any proposal about sequential nature of 

 transcription in microorganismal cells like 

 this. If these cells decide to go back and become 

 another cell type, they're really making dif- 

 ferent antigens which means different genes 

 are being transcribed. On the basis of the biol- 

 ogy of this system it would very unlikely that 

 they would go back and transcribe the whole 

 genome in order. 



LOVETT: Could they go back and start in 

 the middle? 



GROSS: It seems to me it doesn't argue 

 against the sequential transcription so much as 

 it does that it's obligatory that it starts at one 

 end and can't do anything until it reaches the 

 other end, and then starts over again. 



TS'O: That model you have in mind, Paul, 

 must be a linear one and not a circular one. 



GROSS: Yes. 



GREGG: It's hard to say whether they start 

 at the beginning or in the middle. If you examine 

 a cell mass, you see what appears to be a sort 

 of a background fluorescence, and then when 

 you get prespore differentiation in the late ag- 

 gregate, you see spots of prespore antigen. The 

 antigen that reappears in the isolated one appears 

 to be this background antigen which is present 



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