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CAROLINE McGILL. 



began to lengthen but could not stretch out to more than one 

 third the length of the normal expanded Hydra. The Hydra 

 was fixed and sectioned. The sections show distinct cells but in 

 places, especially in the tips of the tentacles, there are marked 

 cytological changes, changes such as might be brought about by 

 a rapid loss of water. 



Experiment 4.. — July, 1907. The Hydra used in this experi- 

 ment was collected at the same time as that used in experiment 3. 

 It was a large brown Hydra with two buds, one large with long 

 tentacles the other small with no tentacles. It was kept at a tem- 

 perature of 4 to 6° C. for six days. At the end of the experi- 



A B 



Fig. 2. A budding Hydra after an exposure of six days to a temperature of 4 ; 

 A, when taken from the ice box ; B, two minutes later when brought to the temper- 

 ature of the room. 



ment it was firmly contracted, Text-fig. 2, A. When the tem- 

 perature was raised it rapidly became active and in a few minutes 

 extended until it appeared as shown in Text-fig. 2, B. It seemed 

 incapable of expanding farther. The Hydra was fixed and sec- 

 tioned. It still had clear cell structure, Fig. 5. 



Experiment 5. — July, 1907. A Hydra taken from a pond 

 at a temperature of 30 C. was kept for seven days at 4°. At 

 the end of the time it was contracted into an oval mass, the 

 tentacles showing only as tiny knobs. When brought to the 

 room temperature it could expand only very slightly, so little, 

 in fact, that some magnification was necessary in order to detect 

 it. When observed under the microscope while still alive no 

 cell layers could be seen. Sections of the Hydra after fixation, 



