SO CAROLINE McGILL. 



During the experiments the Hydra were usually kept in water 

 from the ponds where they had been collected, although they 

 appear to live as well in ordinary tap water. The water was 

 changed frequently since even at low temperatures there is con- 

 siderable evaporation. 



When the Hydra were to be studied histologically they were 

 fixed in an extended condition in a hot corrosive-acetic mixture, 

 run up through the graded alcohols, embedded in paraffin and 

 cut in sections, 57- thick. The sections were stained in haema- 

 toxylin-eosin. 



The series includes about twenty experiments on something 

 like seventy-five Hydra. From this number, five typical experi- 

 ments have been chosen for description. Two of these were on 

 Hydra collected in the winter, three on those collected in the 

 summer. 



Experiment 1. — February, 1907. One well-expanded brown 

 Hydra was placed in an ice box, kept at a temperature of 2° C. 

 The Hydra slowly contracted. At the end of nine days it was 

 removed and appeared then as shown in Text-fig. 1, B. The 

 body and tentacles had contracted until they were about one third 

 the normal expanded length. The body had decreased some- 

 what in volume as if shrinkage had taken place. Upon removal 

 from the ice box the temperature of the water around the Hydra 

 was quickly raised to that of the room. When the temperature 

 reached 8° C, the Hydra began to expand and by the time it 

 had reached io°, the Hydra had stretched to the length shown 

 in Text-fig. 1, C. It had just the appearance of a normal ex- 

 panded Hydra except that the tentacles were slightly shorter and 

 somewhat opaque. When stimulated the Hydra rapidly con- 

 tracted to the size shown in Text-fig. 1, A. Viewed under the 

 low power of the microscope, while still alive it showed appar- 

 ently normal structure ; two distinct cell layers, nematocysts, 

 etc. Ten minutes after its removal from the ice box the Hydra 

 was fixed. Sections of the fixed Hydra showed practically nor- 

 mal cell structure, Fig. 6. The Hydra used in this experiment 

 was taken from a pond having a temperature of about io° C. 



Experiment 2. — November, 1906. A brown Hydra with a 

 small bud was kept at a temperature of 4 to 6° C. for eight days. 



