SCOTT, STUDY OF CHANGES IN MUSTELLS CAN IS 71 



time of death in fresh water. In fact, the change in the osmotic pressure 

 of the blood due to dilution alone would cause a rise in the freezing point 

 of the blood of about 0.30°. Therefore, mere dilution of the blood up to 

 the point at which salts begin to diffuse out would pass the limit in the 

 range in the osmotic pressures of the blood and cause death. This ex- 

 plains why the dog-fish failed to regain the normal freezing point of its 

 blood on return to sea-water after a change of about 0.30° due to immer- 

 sion in fresh water. Because of such a reduction in the osmotic pressure 

 of the blood the constitution of the protoplasmic molecules is disturbed 

 in part, and on the return to sea-water the normal relations fail to be 

 regained. 



Conclusions 



The following conclusions regarding the osmotic relations of MusteJus 

 canis seem to be warranted : 



The osmotic pressure of the blood of the fish varies about an optimum 

 represented by a freezing point of — 1.87°. 



The change in the osmotic pressure of the blood due to changes in the 

 molecular concentration of the external medium depends, 



1st, upon the time of immersion in the external medium, and, 



2nd, upon the modification in the molecular concentration of the ex- 

 ternal medium. 



The change in the osmotic pressure of the blood is not equal, but yet 

 bears quite a constant ratio to the change in the molecular concentration 

 of the external medium. The blood of Squalus living in brackish water 

 has a higher osmotic pressure than that of the water in which it lives. 



When a considerable modification in the osmotic pressure of the blood 

 is brought about by immersion of the fish in solutions hypotonic or hy- 

 pertonic to sea-water, the normal osmotic pressure of the blood is not 

 regained by the return of the fish to sea-water. 



The changes in the osmotic pressure of the blood take place through 

 the gill membranes. 



The osmotic pressure of the blood is not greatly modified by the ab- 

 straction of one-half the total quantity of blood in the body. 



Although the blood is but faintly laked on immersion of the fish in 

 fresh water, the corpuscles are swollen. 



The resistance of the erythrocytes of elasmobranchs to haemolysis is 

 not much inferior to that of the marine teleosts and appears to be inde- 

 pendent of osmotic relations of the corpuscles to its surrounding medium, 

 nor does there appear to be any close relation between the resistance of 

 the corpuscles to haemolysis and the salt content of the plasma. 



