HELEN D. PARK 



369 



^ ^ 10 15 20 25 30 35 40 45 '50 55 60 65 70 '75 80 85 90 95 100 

 DAYS AFTER START OF EXPERIMENT 



Fig. 4. Rhythmicity of sexual differentiation in four clones of Carolina 

 stock Hydra. 0-30 days, ordinates represent total numbers of sexual Hydra; 

 from day 31 on, ordinates represent percentages. • and A , aerated with 

 room air; ■ and ▼ not aerated. 



has been the same as that of the clones (viz. 150); it has never 

 been aerated. The maximum number of sexual forms (7%) was 

 seen on day 34. Since then the culture has been almost free of sexual 

 Hydra; four, four and five were found on each of three days. There 

 have been long intervals (20 days) when no sexual forms could be 

 found. 



In summary, then, under the conditions in our laboratory, the 

 Loomis stock Hydra, both in mass and isolation cultures, have un- 

 dergone alternating sexual and asexual periods during the past 

 three years. Furthermore, four clones of Carolina Biological Supply 

 Company stock have shown a similar periodicity for four months. 

 The original Carolina culture has remained relatively free of sexual 

 Hydra. 



At present we cannot explain the alternating periods we have 



