368 THE BIOLOGY OF HYDRA : 1961 



of the country might be different from the Loomis stock in its pat- 

 terns of sexual differentiation. 



The Carohna Hi/dra arrived in pond water. We were immedi- 

 ately impressed by their large size when compared with the Loomis 

 stock. They were, and have continued to be, three to four times 

 larger. We need to ha\'e the hydra positively identified, but whether 

 they are H. littoralis or not, we have learned something from them. 



The Hyclra were asexual when they arrived. We put 16 of them 

 in 120 ml. of BVT in a small finger bowl giving a fluid depth of 

 about 1.5 cm. This culture received the same care as the Loomis 

 stock, except that no Hydra were discarded until the total reached 

 150 (on day 21). We saw the first sexual Hydra (a male) on day 

 10 when the population was 34. It was isolated for starting a 

 clone. In the next 25 days, as many as 15 males and 2 females were 

 seen at one time. On each of three days, one male was isolated. It 

 is the four male clones I wish to describe now. Each male was put 

 in a 30 ml. beaker as described for the isolation experiments but 

 the population was allowed to increase. When each population 

 leached 15 or 20 the clones were transferred to small finger bowls. 

 When the populations reached 150 they were kept constant by daily 

 random thinning. Two clones were aerated with room air; the other 

 two were partially covered and were not aerated. 



Figure 4 shows the rhythmic nature of sexual differentiation in 

 the four clones. From 0-30 days (while the populations were small) 

 the ordinate values are the total numbers of sexual Hydra. From day 

 31 on, the ordinates represent percentages. Again, as in the cultures 

 of Loomis stock there was a rhythmicity in sexual difl^erentiation. 

 The four curves, on the basis of the number and height of the peaks, 

 can be roughly divided into two pairs: the triangle and circle, and 

 the square and inverted triangle. The triangle and circle show the 

 percentages of sexual Hydra in the cultures that were aerated with 

 room air; the other two show the percentages in the cultures that 

 were not aerated. 



In order that you join me in a state of confusion, 1 want to re- 

 turn to the 16 Hydra culture from which these four clones were start- 

 ed. The culture is now 125 days old. It is not a clone; it presum- 

 ably contains both potential males and females. It has been sitting 

 on the laboratory bench partially covered; the population density 



