tSCOTT, STUDY OF CHANGES IX MUSTELUS CAN IS 69 



have migrated into fresh water. Such a regulative mechanism is one of 

 the mechanisms of adaptation. 



The dog-fishes, on the other liand, are migratory. I think it probable 

 that they are provided with a sensory apparatus by which they are made 

 aware of marked decreases in the concentration of the sea-water, with 

 the result that they avoid dilute media. The dog-fishes are provided in 

 addition with an excretory apparatus which is able to regulate to a modi- 

 fied extent the osmotic pressure of the blood. The result of this activity 

 of the kidneys is that the change in the osmotic pressure of the blood is 

 always less than the change in the external medium. The kidneys con- 

 serve those substances which contribute to the molecular concentration of 

 the blood and eliminate the excess of water. There is a limit, however, 

 to this life-saving action of the kidney. 



The effect of a stimulus depends not only upon its intensity but also 

 upon the suddenness of it. Osmotic changes are induced more rapidly 

 by a sudden than by a gradual change from sea-water to fresh water. In 

 fact, in my experiments a sudden great change in the osmotic pressure 

 of the external medium sometimes caused a rupture of the gill membrane 

 at certain points with a resulting flow of blood. The gradual transition 

 from sea-water to fresh water prevented this bleeding from the gills. 

 Death occurs more quickly in such cases without a great change in the 

 osmotic pressure of the blood. These are simply instances of a wider 

 application of Du Bois Eaymond^s law of stimulation. But it has been 

 shown (p. 28) that the osmotic change occurs through the gill mem- 

 branes. These, however, are not strongly resistant to changes in the 

 osmotic pressure of the external medium. 



The reason that the dog-fish can withstand moderate changes in the 

 external medium is not because it resists these perfectly, but because the 

 organization of its protoplasm is of such a nature that life activities can 

 continue even though the osmotic character of its blood is considerably 

 modified. The heart of Mtistelus continues to beat long after respira- 

 tion has ceased after immersion in fresh water. Squalus and other elas- 

 mobranchs live in the dilute sea-water at the I^ew York Aquarium ; and 

 yet the osmotic pressure of the blood of Squalus, while considerably above 

 that of the harbor water, is still but nine-tenths of that found in fishes 

 living in sea-water. The osmotic pressure of the blood of higher forms 

 never has been proportionately reduced without serious impairment if 

 not cessation of protoplasmic activities. 



Moore ('08), in advancing strong arguments to show the failure of the 

 membrane theory to account for the equilibrium between the cell and its 

 environment, suggested that the cell was able to undergo reversible proc- 



