364 



THE BIOLOGY OF HYDRA : 1961 



impossible to find. This seemed interesting and puzzling enough 

 to warrant quantitative investigation. Therefore, Hydra were picked 

 at random from stock and placed in groups of 400 in finger bowls 

 10 cm. in diameter, containing 150 ml. of BVT. This gave a fluid 

 depth of approximately 2 cm. These cultures were maintained as 

 described above except that before each feeding we made total 

 counts and counts of the sexual forms, and after each feeding we 

 randomly discarded animals to keep the population at 400. 



Figure 1 shows the percentages of sexual forms observed in a 400 

 Hydra culture over 200 days (Cf. 6). The ordinate value of each 

 point is the average of four days' measurements. This culture was 

 kept in the same bowl throughout. For the past 12 months, however, 

 we have been changing all cultures to clean dishes and counting 

 once a week. The percentages of sexual forms observed weekly have 

 ranged between 10 and 55. I think you can see from Figure 1 why 

 we began to think in terms of rhythmicity of sexual differentiation. 



This type of curve is interesting, but tells us nothing about what 



20 



40 



60 80 100 120 -140 



DAYS AFTER START OF CULTURE 



200 



Fig. 1. Rhythmicity of sexual differentiation in a culture of Loomis stock 

 Hydra maintained at constant population density for 200 days. Each ordinate 

 represents the mean of four days observations. 



