312 THE BIOLOGY OF HYDRA : 1961 



ever found is eleven days of age. That's at about 17°. You can 

 find older ones if you lower the temperature. Clijtia, by contrast to 

 Campanularia adjusts in size to the amount of food that is avail- 

 able. In Campanularia you get essentially the same size hydranths 

 regardless of how well or poorly one feeds the colony. If it starts 

 to make a hydranth it makes one of the standard size. Although 

 Clytia hydranths do vary in size they don't grow after they're fully 

 formed. You can get very tiny hydranths if the colony is starved 

 and some hydranths as large as Campanularia if they are well fed. 

 If Clytia is growing on Artemia and for some reason they don't 

 catch their food on a regular basis, they very soon get to a size 

 where they can't ingest Artemia because none of the hydranths are 

 large enough. 



CROWELL: There is some variation in Campanularia. If one 

 uses tissue masses of different sizes, one finds that there is a lower 

 limit where one gets no hydranths. Above that, one gets specimens 

 somewhat smaller than normal and with a smaller tentacle number. 

 Then if one uses still larger masses one gets correspondingly 

 larger hydranths. It's not very striking though. 



STREHLER: There is one implication in a word that you used. 

 You said that there was a zone of "proliferation" down near the 

 developing bud and I just wonder how you would explain certain 

 experiments we did last summer which consisted of giving a colony 

 100,000 r of X-rays, enough so that the slides on which they were 

 growing became deep amber in color. Still, after ten days, a few new 

 hydranths were formed in the radiated colony. Just a few, it's true. 



CROWELL : Subterminal hydranths? 



STREHLER: These were replacements, I believe, i.e. subterm- 

 inal. The point is, that it's hard for me to see how cell division 

 could occur after that amount of radiation. I would propose alter- 

 natively, that there are cells which have aheady divided and 

 which probably lie in the stolon. At the proper signal these cells 

 migrate into the region of what one might call growth, but which 

 I think may better be considered as regions of differentiation and 

 morphogenetic movement where no cell division is taking place. 



CROWELL: I think what you suggest is x^erfectly possible. The 



