324 THE BIOLOGY OF HYDRA : 1961 



8 10 12 14 



WEEKS 



Fig. 7. Relationship of colonial growth and the initiation of medusa bud- 

 ding at different temperatures. Values on the abscissa represent the cumu- 

 lative number of hydranths per colony. Arrows indicate the appearance of 

 the first medusa buds. 



first medusa buds were found to be significant at the 95% level. 



The relationship between the production of frustules and the 

 appearance of medusa buds is illustrated in Figure 8. Colonies at 

 all three temperatures exhibited an early rise in the production of 

 frustules and a later decline. In each case frustule budding began 

 after a decline in the initial rapid production of hydranth buds, 

 and medusa buds appeared immediately following the decline in 

 production of frustules. 



These experiments clearly suggest that hydranth budding may 

 limit medusa budding, since medusa buds always appeared after 

 hydranth budding had declined or stopped. Furthermore, the short- 

 ened phase of hydranth budding at 20° is associated with the 

 earliest formation of medusa buds, while the extended period of 



