26o LOEB. [Vol. VII. 



plasm. I have investigated this point myself, and have caused 

 others also to take up this question. The result of all investi- 

 gations hitherto carried on is as follows : Raising the concen- 

 tration of the salt solution in which an animal or a tissue lives 

 has qualitatively and quantitatively the same effect as lowering 

 the temperature ; lowering the concentration has qualitatively 

 and quantitatively the same effect as raising the temperature. 

 I will mention two cases to illustrate this. First, one example 

 to show the parallelism of the mentioned effect of the tempera- 

 ture and the concentration in qualitative regard. I have recently 

 succeeded in making animals belonging to different classes — 

 larvae of Polygordius, Copepods, etc. — positively heliotropic by 

 bringing them into low temperatures, and making them nega- 

 tively heliotropic by raising the temperature of the water. In 

 water from o° to about io° larvae of Polygordius, for instance, are 

 exclusively positively heliotropic. In water above 25° they are 

 exclusively negatively heliotropic. But by adding a certain 

 amount of NaCl to normal sea-water I was able to make them 

 just as well positively heliotropic, and by adding a certain 

 amount of fresh water to the normal sea-water I could make 

 them negatively heliotropic. The same was the case in Cope- 

 pods, only the absolute figures differ, as was to be expected. 

 To show the quantitative parallelism, I refer to the fact that 

 lowering the temperature diminishes the irritability of the tis- 

 sues. This can easily be shown in the heart. As every one 

 knows, cooling off the heart makes it beat more slowly, while 

 heating it increases the rate of beating. I asked Miss Schively 

 to investigate how the rate of the heart-beat would .depend on 

 the concentration of the solution in which the isolated heart \)r 

 the whole animal is put. Her investigations were extended 

 over the heart of ascidians, crustaceans, embryonic and adult 

 vertebrates, and even the rhythmical motions of medusae ; and 

 she found that the rate decreases with astonishing regularity, 

 especially in the cut-out heart of ascidians, in the measure the 

 concentration of the sea-water by adding NaCl increases ; and 

 that the rate increases with the same regularity by adding fresh 

 water to the normal sea-water. By bringing living tissues into 

 a solution of higher concentration, we therefore reduce their irrita- 

 bility by reducing the amount of water contained in them. By 

 reduction of irritability we mean that the effect determined by 



