SEARS CROWELL 315 



tips being resorbed and regenerating continuously. Since there is 

 no other source of nutrients, old cells must be reused. In line with 

 this, I wanted to emphasize that there is normally no regression of 

 hydranths in Cordylophora. Kinne ( 1956, Zool. Jahrb., Abt. Phy- 

 siol. 66: 565 ) followed individual hydranths for about 140 days, and 

 I have observed them for several months with no indications of 

 regression. 



STREHLER: Does the hydranth continue to get larger during 

 all that time? 



FULTON: They may grow very very slowly. They reach adult size 

 I'd say in about a week of growth. 



CROWELL: You haven't said whether new cells are being pro- 

 duced by using substances derived from old cells, or whether the 

 same cells are producing the new parts by migrating. 



FULTON: I don't know. All I'm saying is that they can't be 

 using up too much because they will go on for months. 



CROWELL: It will go on a long time. 



LYTLE: Or it's a very efficient system for recycling materials. 



STREHLER: It would be very interesting to know whether the 

 same cells stay in the fully formed hydranth if it's not growing. 

 That is, is there a cycle of cell replacement? One should be able to 

 find out by seeing the effect of large doses of X-radiation on the 

 longevity of CorchjJophora. Will it kill them in the same doses 

 which double the longevity of Campanularia? 



SLAUTTERBACK: Every attempt we have made to demonstrate 

 an increased mitotic rate following amputation of hydra heads 

 has been unsuccessful. The formation of a new head with its 

 tentacles appears to be strictly a matter of migration of cells from 

 the column. There is no change in the level of differentiation of 

 these cells nor is there any visible increase in mitotic activity. 



CROWELL: How successful are you in finding mitosis down in 

 the lower region? 



SLAUTTERBACK: We can see them fairly commonly in the in- 



