1910.] 



Blood in Vertebrates and Invertebrates. 



623 



readily be that the renal organs, which do not function in the embryonic life 

 of the individvial, may arise relatively late, and yet in the Eovertebrate 

 embryo have been amongst the very earliest structures to appear. 



It is evident from the analyses of the blood of Limulus that the inorganic 

 •composition of its internal medium is determined by the composition of the 

 ocean. If it were now to develop an excretory organ, with a function like 

 that of the Vertebrate kidney, in its descendants in the far future, many 

 millions of years from now, their internal medium, the blood, woidd in its 

 inorganic composition reproduce in the main the ratios of the sodium, 

 potassium, calcium, and magnesium, and also the saline content of the ocean 

 •of this age, although in that long interval the ocean would undergo very 

 considerable change in composition and in saline concentration. The com- 

 position of the blood in that far remote future could be used to postulate the 

 composition of the ocean of to-day. 



So, from the composition of the blood plasma in Vertebrates, we may infer 

 the relative composition of the ocean in the remote past, when the Vertebrate 

 kidney acquired the function of controlling the salts and their concentration 

 in the blood. This would give for the ocean of that age the ratios in round 

 Talues as follows : — 



Na. K. Ca. Mg. 



100 600 3-00 1-50— 2-00 



Compare this with the ratios of the ocean of to-day, which are : 

 Na. K. Ca. Mg. 



100 3-613 3-911 12106 



This would place the origin of the distinctively Vertebrate type, that is, of 

 a form possessing a kidney with a function like that of the Vertebrate 

 •excretory organ, at a time when the ocean had one-eighth to one-sixth of 

 its present concentration of magnesium, and less than one-third of its present 

 content of sodium cliloride.* 



How far back in time that is to be placed cannot be estimated with 

 certainty, for we do not know what was the sodium chloride content of the 

 earhest ocean of the globe. Joly.f in calculating the age of the earth from 

 the total sodium chloride now in the sea, and from the annual increment of 

 the salt due to the river discharges from the land surfaces of the globe, 

 assumes that it amounted to 1-4 per cent., or approximately one-seventh, that 



* The concentration of sodium chloride in sea water is, in round numbers, 2 '8 per 

 cent., while in mammalian blood plasma it is about O'T per cent. 



t "An Estimare of the Geological Age of the Earth,'" ' Trans. Eoy. Dublin Society 

 Series 2, 1899, vol. 7, p. 23. 



