No. 599] OSMOTIC PRESSURE OF THE BLOOD 649 



greater than that of the external medium. Furthermore, 

 analysis shows that the osmotic pressure of elasmobranch 

 blood is due to different substances from those which ac- 

 count for the osmotic pressure of the blood of marine 

 invertebrates. Therefore the elasmobranchs belong to a 

 second category. In the third group we will place the 

 marine teleosts. The osmotic pressure of their blood is 

 somewhat less than half that of the medium in which they 

 live. We have the case of four species from the Mediter- 

 ranean and thirty-two species from the ocean which show 

 this. The osmotic measurements show a decided differ- 

 ence between the blood and the surrounding medium. A 

 decided independence also. In the same group or pos- 

 sibly a fourth group we will place the fresh-water fishes 

 and with these the amphibians, reptiles, birds and mam- 

 mals. Thirteen species of fresh-water fishes possess blood 

 with an osmotic pressure less than that possessed by the 

 marine teleosts. Let us assume here that the fresh- 

 water fishes were derived from marine ancestors. In be- 

 coming acclimated to fresh water, the blood suffered a 

 decrease in its osmotic pressure. Whether this was in 

 direct response to the great decrease in the osmotic pres- 

 sure of the surrounding medium as compared with sea- 

 water is problematical, but appears probable. The am- 

 pin bians were derived from the fresh-water teleosts. 

 Some of the amphibians still retain their aquatic habits 

 and structures. They in all probability possess the os- 

 motic pressure of fresh-water fishes. Other amphibia 

 metamorphosed into terrestrial forms, taking with them 

 the osmotic pressures of the blood possessed by their fish- 

 like ancestors. Blood with the same osmotic pressure as 

 that of the fresh-water fishes flows on through the am- 

 phibia to the reptilia and on to the birds and mammals. 

 An examination of Table II shows the close similarity 

 between the osmotic pressures of fresh-water fishes, am- 

 phibians, reptiles, birds and mammals. According to the 

 above hypothesis, the order of evolution was I. Marine 

 invertebrates, II. Elasmobranchs, III. Marine teleosts, 



