580 THE COLORING MATTER OF BLOOD 



the blood of different animal species vary considerably. As 

 a rule they form needles, prisms and plates of rhombic 

 system ; the hexagonal crystals of the squirrel and the tetra- 

 hedra of guinea-pig haemoglobin are well known. Formerly 

 much importance was made of such differences. It has been 

 shown, however, that by repeated recrystallization one form 

 of crystal can be converted into another ; 2 that therefore 

 we are dealing with a typical heteromorphism which may 

 be looked upon as an expression of a certain molecular 

 instability. When we take up the question of the chemical 

 identity of haemoglobin, however, there are a number of 

 other factors to be taken into consideration. In the first 

 place it should be remembered that even by repeated re- 

 crystallization it is not always possible to exclude with 

 certainty the admixture of other high molecular substances. 

 Besides it is an important point (confirmed particularly by 

 the studies of Barcroft and his collaborators) to remember 

 that the dissociation of oxyhaemoglobin into haemoglobin 

 and oxygen is subject to an influence from the salts and 

 the carbonic acid in the surrounding medium and of course, 

 too, from numerous other factors. 3 We know, moreover, 

 that haemoglobin is an extremely labile substance, the pro- 

 tein component of which may undergo distinct changes even 

 from being left exposed and from drying (Nencki's "para- 

 hemoglobin") without this being necessarily evinced by 

 any important change of form or even any loss of refractive 

 power. And, finally, attention has been directed by the 

 investigations of H. Aron and Franz Miiller 4 to the point 

 that differences between the behavior of blood solutions 

 and pure haemoglobin solutions may be caused by partial 

 conversion of oxyhaemoglobin into methaemoglobin (vide 

 infra), this being at times indeed met even in normal blood. 



2 M. Uhlik (Innsbruck), Pfliiger's Arch., 104, 64, 1904. 



a Barcroft and M. Camis (Physiol. Instit., Cambridge), Jour, of Physiol., 

 39, 118, 1909. 



4 H. Aron (N. Zuntz's Lab.), Biochem. Zeitschr., 3, 1, 1906; H. Aron and 

 F. Miiller (N. Zuntz's Lab.), Arch. f. (Anat. u.) Physiol., 1906; suppl. 110. 



