Light in the Regeneration of Hydroids 137 



the third day. In Experiment 2,20 controls regenerated 14 and 

 8 hydranths on the second and third days respectively, while 42, in the 

 dark, regenerated 4 on the second and 10 on the third day. The 

 same relation obtains in Experiment 3. In Experiment 4, 50 

 controls regenerated i, 2, 10, 5, 4, and 2 hydranths on the 5th, 

 6th, 7th, 8th, 9th and loth days respectively, while the 50 in the 

 dark regenerated o, i, i, 6, 7, and 7 hydranths in the same time. 



The curve representing the number of hydranths regenerated 

 on successive days, are analogous and that given for Eudendrium. 

 The maximum number produced at any one time appeared within 

 a few days after the beginning of the experiment, and the num- 

 ber steadily decreased on the following days until no more were 

 produced. In Experiment 3, no more hydranths appeared after 

 the loth day, in Experiment 2, after the 13th day, in Experiment 

 4, after the 14th day, and in Experiment i, after the 15th day. 

 Though the observations were continued for over 25 days the con- 

 trols no longer produced any hydranths. In this respect Tubu- 

 laria crocea behaves exactly like Eudendrium. 



While the controls no longer regenerated after 7, 10, 13 and 14 

 days respectively, those kept in the dark produced new hydranths 

 during 10, 13, 8 and 17 days for the corresponding series. This 

 difference in time, namely, 3 days, was much greater than the 

 initial retardation. This may be interpreted to mean that the 

 prolonged darkness continued to retard the development of 

 the hydranths. 



Beside this retardation, the removal of light also inhibits develop- 

 ment to a considerable degree. Even during the first cycle, 

 while the number of hydranths that w^ere regenerated was large, 

 the maximum (maximum per cent rather than the absolute num- 

 ber is here referred to), for any one day was always less than 

 among the corresponding controls. Also the total per cent regen- 

 erated during a definite period, 13 days for example, was about 

 31 per cent less in the dark, than in the light. Darkness then 

 exerts certain definite and measureable effects even during the 

 first period or cycle, and in these respects the behavior of this 

 hvdroid is not unlike that of Eudendrium. 



But it is only after this first cycle, that the stalks kept in the dark 



