370 THE BIOLOGY OF HYDRA : 1961 



observed. In our current approaches we are considering both en- 

 vironmental factors, and factors which may reside in each hydra 

 itself. 



REFERENCES 



1. Brien, p. The fresh water hydra. Amer. Scientist. 48: 461-475. 



2. Ito, T. 1952. Studies on the reproduction of hydra. III. Sexual periodicity 



found in the hydra, Hydra magnipapiUata Ito. Memoirs Ehime Univ., Sect. 

 H (Sci.). 1: 221-230. 



3. LooMis, W. F. and H. M. Lenhoff. 1956. Growth and sexual differentiation 



of hydra in mass culture. /. Exp. Zool. 132: 555-574. 



4. LooMis, W. F., 1957. Sexual differentiation in Hydra: Control by carbon dioxide 



tension. Science 126: 735-739. 



5. LooMis, W. F., 1959. In Cell, Organism and Milieu, XVII Growth Symposium. 



The Ronald Press Company, New York, N. Y. pp. 253-294. 



6. Park, H. D. 1959. Sexual cycles in Hydra. Anat. Rec. 134: 623. 



7. Park, H. D., C. Mecca and A. Ortmeyer, 1961. Sexual differentiation in Hydra 



in relation to population density. Nature (in press). 



DISCUSSION 



FULTON: Did you run all of the isolation cultures simultaneously? 

 Or when you were all through did you put them together as 200 

 days? 



PARK: All of the isolation cultures in Figure 2 were run simul- 

 taneously. In Figure 4, I put all of the zero day clones back together. 

 As you can tell by the ends of the lines, we can calculate how 

 far apart in time they were started; I think not less than 3 days 

 nor more than 10 days. 



FULTON: Do these fit together better if you put them in chrono- 

 logical time? I think it would be good to look in terms of periods in 

 the laboratory. In other words, what's happening on December 10th, 

 and so forth, as opposed to time in culture? 



PARK: I understand what you're asking, and I can't say that I 

 know, because what we've seen is rather confusing. Sometimes they 

 fit better if they are put together chronologically. For instance, 

 if we take 10 Hydra plus another 10 which were run six months 

 later, plus another, and put them all back to zero, the periodicity of 



