i8 9 6. THE PIGMENTS OF ANIMALS. 97 



We have already seen that in vertebrates haemoglobin is constantly 

 undergoing decomposition. As to the fate of the products of its 

 retrogressive changes, there is reason to believe that the proteid 

 constituent and the iron are re-employed in metabolism, while the 

 pigment itself (haematin) undergoes various modifications. Thus the 

 pigments of bile, bilirubin, biliverdin, etc., are almost certainly 

 derived from the haemoglobin of the blood; in Amphioxus, where 

 haemoglobin is absent, there are no bile-pigments (Hoppe-Seyler). 

 Under ordinary circumstances, the bile-pigments seem to be reduced 

 in the small intestine, and reappear in the faeces as hydrobilirubin 

 (stercobilin). Urobilin and haematoporphyrin or iron-free haematin, 

 other derivatives of haemoglobin, are excreted in small quantities in 

 the urine. Thus, so far as our present knowledge extends, the 

 pigments derived from haemoglobin do not under ordinary circum- 

 stances play an important part in producing coloration. The only 

 notable exception is, perhaps, the pigment of the skin in the dark races 

 of mankind, which is said to be derived from haemoglobin. Under 

 diseased conditions, on the other hand, the derivatives of haemoglobin 

 may produce marked pigmentation. Thus in jaundice the skin and 

 conjunctivae are stained with bile-pigment ; in Addison's disease, or 

 after removal of the supra-renal bodies, the skin is coloured with 

 pigment which is said to be derived from haemoglobin. 



Again, although the derivatives of haemoglobin are, in vertebrates, 

 unimportant for the individual, they may have some importance for the 

 species. Thus, according to Wickmann (17), the pigments of the eggs 

 of birds are directly derived from the pigment of the blood which fills 

 up the corpus luteum. In the covpus luteum the blood stagnates and 

 undergoes retrogressive metamorphoses which result in the formation 

 of derived pigments. This may be compared to a similar process in 

 mammals, where haematoidin crystals are formed in the corpus luteum ; 

 the difference may, perhaps, be explained by remembering the 

 difference in size of the eggs of birds and mammals, and the conse- 

 quent diminished outflow of blood in the latter. The pigments of 

 birds' eggs are thus related to bile-pigments in that both are derived 

 from a similar source, i.e., haemoglobin. Wickmann further states 

 that the pigments after their formation in the ovary are shed into the 

 oviduct, in the uterine portion of which they are mingled with the 

 materials of the shell. The differences in the pigments of different 

 eggs may, he suggests, be due to differences in the composition of the 

 blood itself. It is well known that in mammals the blood varies in 

 different species, as is shown by the differences in the shape of the 

 crystals of haematin, the colour of the plasma, and so on ; similar 

 differences may express themselves in birds as differences in the 

 products of decomposition. 



As to the occurrence of derivatives of haemoglobin among inverte- 

 brates we have only a few isolated observations, mostly due to Dr. 

 MacMunn (10), and largely dependent on the use of the spectroscope. 



