THE EFFECT OF LOW TEMPERATURES ON 

 HYDRA. 



CAROLINE McGILL, 

 Instructor in Anatomy, University of Missouri. 



In his experiments on Hydra, Greely, '03, observed that when 

 the temperature is reduced, there occurs, what he termed, a re- 

 versal of vital phenomena. The body is quickly reduced to a 

 resting stage, to an undifferentiated mass of protoplasm. He 

 describes the changes as follows : " Whenever a Hydra is ex- 

 posed to a temperature of 4 to 6° C, the tentacles gradually be- 

 come shorter and thicker, and are finally completely absorbed 

 into the body. As the absorption goes on, the ectoderm and 

 endoderm cells of the tentacles lose their individuality and form 

 an undifferentiated mass of protoplasm which is slowly taken into 

 the body of the Hydra. The tentacleless body of the Hydra 

 becomes slowly resolved into a dense spherical mass of coagu- 

 lated protoplasm, in which no distinction between the individual 

 cells can be made out, and remains in this condition as long as 

 it is kept at a low temperature, but quickly forms tentacles and 

 a double layer of cells again when it is returned to the tempera- 

 ture of the room. If Hydra in the earlier stages of the process 

 of budding be placed at a temperature of 4 C, not only does 

 the growth of the bud stop instantly but absorption of the bud 

 into the body of the parent commences, and continues until all 

 traces of the bud have disappeared. . . . Six or seven days are 

 required for the complete disappearance of the bud. . . . Lower- 

 ing the temperature brings about a reversal of vital phenomena 

 and the formation of simple resting stages." 



Greely, 'oi-'o2, in an earlier series of experiments traced in 

 protozoa a similar reversal of vital phenomena brought about by 

 reducing the temperature. He found that when the temperature 

 is lowered the protoplasm of unicellular forms coagulates, with 

 accompanying loss of water and the cells pass into resting stages. 

 His experiments on Hydra led him to conclude that similar 

 changes may take place in metazoa. 



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