334 THE BIOLOGY OF HYDRA : 1961 



LYTLE: No. Unfortunately the conditions in these two experi- 

 ments were not precisely the same, but I do think we can say that 

 temperature is the most important variable here. The 20° cultures 

 were maintained in running water in a constant temperature bath 

 controlled ±1°. The 19-23° cultures, however, were maintained in 

 running water on a water table at the temperature of the incoming 

 water. During the course of this experiment the temperature rose 

 gradually from an initial 19 to 23 and slowly returned to 19° at 

 the end of 16 weeks. There was also a small diurnal variation in the 

 temperature, in the order of about 1 ". Furthermore, because of a 

 technical difficulty there was some difference in the rate of flow be- 

 tween the 20° experiment and the 19-23° experiment, but I doubt 

 that this had any great influence on our results. I believe that the 

 gradual rise and decline of temperature was probably more impor- 

 tant than the small difference in rate of flow or the actual difference 

 in mean temperature between the two experiments, but this has 

 to be investigated further. 



FULTON: I see that your absolute numbers were a lot bigger 

 there. 



LYTLE: Definitely. The large colony with 22 hydranths which I 

 showed at the beginning of my talk was grown on the water table 

 with the rise and fall of temperature (19 -23° -19"). I have never 

 gotten colonies this large in cultures grown under more closely con- 

 trolled temperatures within this range. 



HAND: If I understood your summary, it sounded to me as if you 

 were saying something backwards. You showed that when hy- 

 dranth production falls off, frustule production comes on; and when 

 frustule production falls off, medusa production comes on. It sound- 

 ed as if you were saying that there was a backward action, that the 

 second phenomenon was somehow affecting the first one. What 

 were you thinking about? 



LYTLE: As I stated in my talk, there appears to be a definite 

 hierarchy among the three budding processes. Hydranth budding 

 has first priority, and it is only after hydranth production slows 

 down that frustule production begins. Medusa budding does not be- 

 gin for some time after hydranth budding has ceased or greatly 



