No. 599] OSMOTIC PRESSURE OF THE BLOOD 



059 



marine teleosts in low percentage of organic solutes and 

 this characteristic is maintained by all the higher forms. 

 Dakin found that the blood of the plaice at Helgoland con- 

 tained 0.92 per cent, salts, while at Kiel in brackish water 

 it had a salt content of 0.85 per cent. Mosso ( '90) stated 

 that marine teleost blood had a higher salt content than 

 that of fresh-water forms. Dakin ( '08) found the blood 

 of the eel in sea water to contain 0.605 per cent, salts, 

 while in fresh water its saline content was 0.466 per cent. 

 Quinton ( '00) found that the blood of fresh-water teleosts 

 contained 0.7 per cent, salts. Atwater ( '91) found that 

 the flesh of fresh-water teleosts contains less salt (15 per 

 cent, less chlorine) than that of marine teleosts. Sumner 

 ('05) obtained a similar result. 



The anadromous fishes possess blood that is less saline 

 in fresh water than in sea water. It is also true that 

 strictly marine teleosts of the present day vary a little in 

 the saline content of their blood when the salinity of the 

 external medium changes. These facts indicate that the 

 decreased salinity of the blood of fresh-water teleosts was 

 brought about in response to the low saline content of the 

 external medium. During the migrations that took place 

 in the past when there were probably more anadromous 

 fishes, this diminution in salts took place. Those forms 

 that remained in fresh water retained the percentage of 

 salts they acquired by their sojourn in fresh water. At 

 the same time they built up membranes which maintain 

 an equilibrium in spite of the differences in the osmotic 

 pressure of the blood within and the fresh water without. 

 Similar membranes were formed in case of the marine 

 teleosts, which maintain an equilibrium with the sea water 

 in spite of the fact that the osmotic pressure of sea water 

 is over twice that of the teleost blood. The evidence at 

 hand indicates that the last membranes to become prac- 

 tically impermeable to salts were the gill membranes. 

 And yet though impermeable to salts they still are re- 

 quired to be permeable to gases. 



Now the blood of amphibia contains about 0.7 per cent. 



