314 THE BIOLOGY OF HYDRA : 1961 



SLAUTTERBACK: This intrigues me very much because we've 

 come upon dedifferentiation and redifferentiation in the pedal disc. 

 If one amputates the pedal disc, the secretory cells are soon re- 

 placed but not from the undifferentiated interstitial cell as might 

 be expected, but by partial dedifferentiation of cnidoblasts fol- 

 lowed by differentiation into secretory cells. This observation is 

 possible because the nematocysts persists in these cells throughout 

 the process. In fact, the mature secretory cells often contain a part- 

 ly disintegrated nematocyst. Furthermore, even the organelle devel- 

 opment characteristic of the cnidoblast persists for a time after the 

 secretory cell, with its very different organelles, has begun to func- 

 tion. I think this is one of the rarer demonstrations of a partial de- 

 differentiation and then redifferentiation of the same cell into an 

 entirely different cell line. I wonder if that is what is going on in 

 your situation, or whether you have mitosis intervening, or what? 



CROW ELL: If one starts with a little colony consisting of a stolon 

 and a few hydranths, and does not feed it, one often finds that there 

 is new growth of the stolon and then production of new hydranths 

 from the new stolon. I have seen this in Campanularia and Cor- 

 dylophora; Berrill has described it. Of course, as new stolon and 

 hydranths are growing at one end, old hydranths and stolon are 

 regressing at the other end. One does get regression of hydranths 

 of Cordylophora in this situation; however, there is no regression 

 in well fed colonies. Of course, such a system gradually gets small- 

 er—as long as it lasts it produces new parts at the expenses of 

 the old. 



SLAUTTERBACK: I wonder whether there is a degradation of 

 cells followed by reuse of the degraded material to make new cells, 

 or whether there is a dedifferentiation, migration and redifferentia- 

 tion of the original cells from the old hydranths. 



CROW ELL: That is just the point that is not understood. 



SLAUTTERBACK: In the pedal disk it is the old cells that are 

 reused, i.e. redifferentiated. 



FULTON: This must also be the case in Cordylophora because 

 the stolons of a starving colony will keep extending over the slide 

 for months; the hydranths and stolon tissue behind the advancing 



