190 HEMOGLOBIN 



things, is the only substance which is capable of forming a compound 

 with reduced haematin, which has the significant property of haemo- 

 globin, to wit, the possibility of a dissociable oxygen compound. 



Until recently one had supposed that the great "find" which 

 nature had made was haematin, whilst it was a matter of quite 

 secondary importance that the haematin had been united to a protein, 

 but no one suggested that it was a matter of any particular im- 

 portance what protein was involved. I will not say that the tables 

 are now turned, because haematin does seem to be very much better 

 for its purpose than anything else, but one must point out the 

 difference between haemoglobin and chlorocruorin, another iron-con- 

 taining pigment which is analogous to haemoglobin. This pigment is 

 found in certain worms and was first studied by Ray Lankester; 

 lately it has been the subject of extended research by H. Munro 

 Fox. 



Chlorocruorin has a complete set of derivatives (to use the hackneyed 

 phrase, which seems to infer a logical precedence of the cart to the 

 horse) ; it has compounds analogous to haematin, to haemochromogen, 

 and presumably the relation of chlorocruorin to chlorocruorochromo- 

 gen is the same as that of haemoglobin to haemochromogen, and in 

 each case the change accounts in the same way for the dissociability 

 of the oxygen. So far as I know the protein moiety has not been 

 investigated, but presumably it is a globin. 



The essential difference between chlorocruorin and haemoglobin is 

 to be found in the haematin from which the chromogen is derived. 

 In each case the haematin is an iron-containing compound, and in 

 each case the iron may be split off leaving a porphyrin, but the two 

 porphyrins are different. The point I wish to emphasise is that this 

 difference in the porphyrins only affects the molecules quantitatively. 

 Qualitatively their properties are the same. It is probable that this 

 quantitative difference is enough to make chlorocruorin unsuited to 

 the needs of mammalian metabohsm, so chlorocruorin has, as it 

 were, dropped out of the hunt; the difference in the porphyrins is 

 insufficient to upset the whole properties of the molecule. 



But one may go further and dispense with the iron altogether 

 without essentially changing the molecular character. Haemocyanin 

 possesses the same general properties as haemoglobin, but in a very 

 restricted degree; for which reason the molluscs, in so far as they 

 preferred haemocyanin to haemoglobin, took a course which confined 

 their development to the level of the octopus. 



