328 



THE BIOLOGY OF HYDRA : 1961 



The animals represented by the filled chcles were fed every 

 third day. These animals demonstrate that the rate of hydranth bud- 

 ding is not significantly decreased by the lowered nutritional level 

 but that frustule production is differentially affected. Therefore, 

 this experiment provides direct evidence of a physiological interac- 

 tion between medusa budding and frustule budding and further 

 indicates that this interaction is at least partially nutritional. 



Another experiment with different nutritional levels further illus- 

 trates the interactions between these three morphogenetic processes. 

 Colonies were reared at 23° from frustules taken from low tempera- 

 ture stocks until several hydranths had been formed. These colonies 

 were starved for approximately four weeks to deplete their nutri- 

 tional reserves and were divided into three groups fed at different 

 rates. As indicated in Figure 10, the production of hydranths showed 



10 



n/2 Daily 



I Worm/ 2 Days 



4 5 



WEEKS 



Fig. 10. Colonial growth at three different feeding rates (n = the number 

 of hydranths per colony at the time of feeding). Cultures maintained at 

 23° (±1°). 



