HELEN D. PARK 



367 



intimately related to the presence of other organisms in the dishes. 



Before going on to describe some of our most recent work, I 



want to mention our experiences with clones of the Loomis stock 



EXP I 



Hydra DISHES CHANGED DAILY 



^ ^^ mssi. fsm 



Hydra 

 #7| 



Expn 



DISHES CHANGED DAILY 



*im^///y^m^. 



DISHES NOT CHANGED 



#9| 



I 1 



DISHES NOT CHANGED 



*^ ^t/yMy///.mm'yy//y/////////y^^^^^ #12^ 



10 20 30 40 50 60 70 80 90 100 



DAYS AFTER ISOLATION 



10 20 30 40 50 60 70 80 80 100 



DAYS AFTER ISOLATION 



Fig. 3. Comparison of sexual periodicity in individual, isolated Hydra left 

 in same dishes for 100 days with perodicity in comparable Hydra changed to 

 clean dishes daily. Solid sections of bars, sexual periods; hatched sections, 

 asexual periods. 



transferred from BVT to Daniel-Park saline. Over the past three 

 years we have started, at intervals, perhaps a dozen such clones. 

 All have become sexual after 15-25 days. Our present clone has 

 now been maintained for about two years and the weekly counts 

 show that the percentages of sexual forms vary between 23 and 68, 

 percentages that are quite comparable to those for the cultm'es in 

 BVT. We are unable to explain the difference between the asexual 

 clone of Hydra littoralis in Daniel-Park saline from 1950-1956 

 and the clone of Loomis stock over the past three years. 



About five months ago I decided that we had worked exclusively 

 with the Loomis stock of Hydra long enough. We, therefore, 

 ordered a culture of H. littoralis from the Carolina Biological Sup- 

 ply Company, thinking that perhaps H. littoralis from another part 



