in the vegetative amoebae and easily observed 

 in the early aggregate. So, the background 

 antigen appears first; and this is followed up 

 by the synthesis of the prespore antigen. I 

 doubt if the cells would synthesize prespore 

 antigen in the absence of this background anti- 

 gen. I think the isolated prestalk cells try to 

 establish conditions as they were in the early 

 stages of normal development as a prelude to 

 differentiating into prespore cells. 



EPEL: Relating to Paul's point, maybe 

 this is a difference between microorganisms 

 and metazoans; if this is a microorganism. 



GREGG: It is claimed taxonomically by 

 both the botanists and zoologists. 



GROSS: Are you referring to the capacity 

 for dedifferentiation? 



EPEL: Yes. Is this like a bacterial spore 

 or protozoan, which forms a spore under cer- 

 tain environmental conditions? 



DEERING: Can you take these things after 

 you've cut them once, have them change, and 

 cut them again and have them change back again? 



GREGG: If you don't wait too long, I should 

 think you could. 



DEEFUNG: In other words, you can change 

 5 to a and then back to b? I wonder how long you 

 could keep this up? 



GREGG: Probably until you get down to a 

 very few cells. 



KAHN: A good deal of our discussion has 

 centered around cell metaplasia, the ability of 

 a cell to exist in different states. Recently, 

 Dr. Lindsay Olive (Columbia University) de- 

 scribed an amoeboid microorganism that is 

 capable of assuming amoeboid, flagellate, cyst 

 or spore form. It would be very interesting to 

 know whether this organism can make these 

 transformations in the absence of cell division. 



MASSARO: Isn't it possible, let us say, that 

 certain of these cells in a particular area are 

 like reserve cells, not being particularly com- 

 mitted at any one time to any one tissue; and 

 these cells perform the reorganization? 



LOVETT: I don't think it's necessary to 

 assume that the cells in layer X are identical 

 to the cells in layer Y; but cells in layer X may 

 be dedifferentiated, undifferentiated, or less 

 differentiated cells which are in reserve to be 

 committed to the reorganization or formation 

 of the structure. 



GREGG: You mean this is the case just in 

 the event someone comes along and cuts one in 

 half? 



LOVETT: Certainly. I respect the potential 

 of these cells. 



GREGG: With regard to your remarks I 

 can only say this: in D, mucoroides we've 

 thought about this to a certain extent. Prestalk 

 cells have to be continually replaced by the 

 prespore cells as the slug crawls along because 

 the prestalk cells are continually forming stalk. 

 So in order to keep the proportions of these 

 cells constant it has to keep replenishing the 

 prestalk cells. Now, there appears to be a 

 gradient of differentiation between these two 

 regions. In other words, the further anterior 

 you cut, the more apt you are to get fewer spore 

 cells and more undifferentiated cells following 

 a reorganization period. The closer you cut to 

 the prestalk-prespore junction the greater num- 

 ber of cells you get which have just crossed 

 the border into the prestalk area. Consequently, 

 it's much more likely that they can dedifferen- 

 tiate to form prespore cells. I don't know 

 whether this answers your question about re- 

 serve cells or not. 



DEERING: Can we really eliminate the 

 possibility that there is a third type of cell that 

 can go either way and that this is what always 

 leads to appearance of new types? 



GROSS: I think you can. 



GREGG: I suppose it would be possible. 



DEERING: In other words, you can't really 

 eliminate that possibility. I think it' s important. 



GROSS: But the requirement is that if you 

 had such a population of cells, they would have 

 to be uniformly distributed throughout the slime. 



MASSARO: Why is it necessary to have a 

 uniform distribution? 



GROSS: Because you get regulation wher- 

 ever you cut. If they were restricted to one end, 

 then you wouldn't get regulation at the other 

 end. 



LOVETT: However, don't you get varying 

 degrees of reorganization depending on where 

 you cut? 



GREGG: Yes, fruiting body formation in- 

 variably occurs although the proportions of the 

 two types of cells composing it depends upon 

 where you cut and the amount of time the pre- 

 stalk mass requires to reorganize. 



LOVETT: All pieces of the slug cut at the 

 proper stage will eventually reorganize and 

 regulate the proper proportion between prestalk 

 and prespore cells? 



GREGG: Presumably if they're allowed to 

 migrate long enough they reestablish their pro- 

 portions. I think one of the reasons that you did 

 not see prespore cells immediately in the an- 

 terior tips that I showed you was due to the fact 

 that the anterior tips rush right into fruiting; 



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