No. 3.] 



THE REGENERATION OF HYDRA. 



127 



At room temperature, i.e., 18-24° C., the regeneration is 

 much slower. In five experinients, in whicii nineteen polyps 

 were injured, there was no regeneration completed at forty- 

 eight hours ; and at the end of seventy-two hours a very small 

 number were complete, as Table 4 shows. 



Table 4. — Hydra Grisea. 



Temperature. 



18-24° 



Per Cent Regenerated. 

 96 hrs. 



48 hrs 



0% 



72 hrs. 

 26.3% 



94-7% 



In order to show the great difference in the rate of regen- 

 eration of the two species and the effect of the higher tem- 

 perature, a record, in which several sets of experiments are 

 combined, is given in Table 5. 



Table 5. — Comparison of H. Grisea and H, Viridis. 



Owing to lack of material, I was unable to try the effect of 

 lower temperatures on Hydi'a grisea. The results obtained 

 from a series of five experiments on Hydra viridis show that 

 when the polyps are subjected to cold, regeneration is greatly 

 retarded. Table 6 is the record of thirty-eight individuals at a 

 low temperature where the thermometer was kept at 12° C. 



Table 6. — Hydra Viridis. 



