COMPOSITION OF THE MINERAL CONSTITUENTS 269 



of KC1 to 2 molecules of CaCl 2 , the total concentration being one-sixth 

 molecular.' 



Not only, however, are the mineral constituents of mammalian 

 serum constant in composition, but even in the blood of fishes we find 

 that substantially the same relative proportions obtain. The following 

 analyses are cited after Macallum, the percentage-concentTSition of 

 sodium being taken as 100 and the percentages of the remaining metals 

 reduced to the same units: 



Species. Na K Ca Mg 



Dogfish (A canthias vulgaris) .100 4.6 2.7 2.5 



Cod (Gadus callarias) ... 100 9.5 3.9 1.4 



Pollock (Pollachius mrens) . . 100 4.3 3.1 1.5 



Dog 100 6.9 2.5 0.8 



Mammal (average) .... 100 6.7 2.6 0.8 



Man 100 6.1 2.7 0.9 



The remarkable uniformity of composition which is thus displayed 

 by the blood-sera of such diverse organisms, suggests that it is deter- 

 mined by some common cause, more especially since a slight alteration 

 of the normal mineral composition of blood-sera causes profound 

 disturbance of the functions of the tissues. The interesting suggestion 

 has been put forward by Macallum that the mineral composition of 

 vertebrate blood-sera represents the composition of the sea-water at the 

 time when the early ancestors of the present vertebrate forms first 

 acquired an organ, namely the kidney, of which the function is to 

 maintain constancy of composition in the body-fluids. In the lower 

 marine forms of the present day which do not possess any corresponding 

 excretory organ, the composition and concentration of the body-fluids 

 is practically identical with that of the sea-water in which they live, 

 but in mammals the tissue-fluids are not only more dilute than present- 

 day sea-water, but they differ from it in containing a much smaller 

 proportion of one mineral constituent, namely Magnesium. The fol- 

 lowing figures are cited after Macallum, the percental-concentration 

 of sodium, as before, being taken as 100 and the percentages of the 

 remaining metals reduced to the same standard. 



Fluid. Na K Ca Mg 



Ocean-water 100 3.6 3.9 12.1 



Tissue-fluid of a jellyfish (Aurelia 



flavidula) 100 5.2 4.1 11.4 



Blood-serum of a dog ... 100 6.9 2.5 0.8 



The correspondence of the three sets of figures, excepting in regard to 

 magnesium, is certainly striking and the oceanic origin of these widely- 

 found ratios appears very probable. 



Among the crustaceans the more primitive forms, such as Limulus 

 possess a blood-serum which is practically of the same composition as 

 sea-water. In more highly developed forms such as Homarus an 



1 The actual ratios in Locke's solution are 100 NaCl : 3.6 KC1 : 1.4 CaCl?, In sea- 

 water the ratios are: 100 NaCl : 2.2 KCJ : 1.5 CaCl?, 



