Apparent Rhythmicity 



in Sexual Differentiation 



of Hydra littoralis 



Helen D. Park 



Laboratory of Physical Biology, National Institute of Arthritis and Metabolic 

 Diseases, National Imtitiites of Health, Bethesda 14, Maryland. 



Hydra have been studied in our laboratory for many years, first 

 by Dr. Harold Chalkley, then by Dr. George Daniel, and for the 

 past seven years by myself. Dr. Loomis' early studies (3, 4, 5) on 

 Hydra littoralis, which he has just reviewed for us, are especially 

 interesting to me because Dr. Daniel and I maintained mass cul- 

 tures of a clone of Hydra littoralis in our laboratory from 1950-1956 

 with only one three-week period in which any sexual Hydra were 

 observed. These cultures were maintained in a solution containing 

 iOO mg. NaCl, 4 mg. KCl, 48 mg. CaCL per liter of double dis- 

 tilled HoO, the last distillation being from glass. The cultures, 

 unfortunately, were discarded in 1956. 



In 1958, however, we obtained a culture of H. littoralis from Dr. 

 Edward Kline of the Armed Forces Institute of Pathology. 

 These were Loomis stock and had been cultured in BVT ( 100 mg. 

 NaHCOs and 50 mg. Versene per liter of tap H.O ) . We established 

 a clone from a spermary-bearing individual and for three years 

 have maintained in BVT mass cultures derived from this single 

 male. Daily except Sundays we allow the cultures to feed on an ex- 

 cess of Artemia lar\'ae for 30-60 minutes, rinse them with tap H^O, 

 and replenish the BVT. The laboratory temperature is 24° ± 2°. 



Soon after setting up the clone, we noticed that on some days 

 sexual forms were abundant; on other days they were difficult, if not 



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