CHAPTER II 

 THE EXTERNAL FACTORS OF REGENERATION IN ANIMALS 



THERE is a constant interchange of material and of energy that 

 takes place between a plant or an animal and its surroundings, and 

 this interchange may be influenced by such physical conditions as 

 temperature, light, gravity, etc., or by such chemical conditions as 

 the composition of the atmosphere or of the water surrounding the 

 organism. We can study the process of regeneration either by keeping 

 the regenerating organism under the same conditions that it is subject 

 to in its natural environment, or else we can change the surrounding 

 physical or chemical conditions. In this way we can determine how 

 far the regeneration is affected by external changes, and how far it is 

 independent of them. If a change in the external conditions pro- 

 duces a definite change in the regeneration, then the new condition is 

 called an external factor of regeneration. 



TEMPERATURE 



That the rate at which regeneration takes place can be influenced 

 by temperature has been shown by Trembley, Spallanzani, Bonnet, 

 and by many more recent writers. In fact, so familiar is the process 

 to every one who has studied regeneration, that it is usually taken for 

 granted that such is the case. 



In general it may be stated that the limits of temperature under 

 which normal growth may take place represent also the limits of 

 temperature for regeneration. Lillie and Knowlton ('97) have deter- 

 mined the limits of temperature within which regeneration takes 

 place in Planaria ton>a. The worm was cut in two transversely 

 through the pharynx, and the time required at different temperatures 

 to produce a new head on the posterior piece was recorded. The 

 lowest temperature at which regeneration was found to take place 

 was 3C. Of six individuals kept at this temperature only one regen- 

 erated at all, and in this one the eyes and brain were still incomplete 

 after six months. The optimum temperature, or at least that at 

 which regeneration takes place most rapidly, was found to be 29. 7 C.; 

 a new head developed in 4.6 days at this temperature. At 3i.5C. 



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