2S8 COLOUR IN NATURE chap. 



4. Yellow ooxanthine — a bright yellow pigment giving rise 



when mixed with oocyan to the bright permanent 

 green so familiar in the eggs of the emu. 



5. Rufous ooxanthine — a reddish-yellow pigment perhaps 



peculiar to the eggs of the tinamou. 



6. A substance giving a banded spectrum but otherwise 



little known. 



7. Lichenoxanthine — a brick-red pigment, possibly due to 



the growth of minute fungi. 



As to the nature of the pigments, Krukenberg 

 regards the blue and green colours as due to modi- 

 fications of the bile - pigment biliverdin, and the 

 brown and red colouring-matters as closely allied to 

 iron-free hsematin (hsematoporphyrin). A more recent 

 observer, Wickmann, regards all the pigments as 

 originating directly from haemoglobin. According 

 to him, the pigments originate from the blood which 

 fills up the corpus lutetim. This blood stagnates 

 and undergoes retrogressive metamorphoses which 

 result in the formation of the pigments. He com- 

 pares the process to that occurring in mammals, 

 where there is a formation of hsematoidin crystals in 

 the corpus luteum ; the difference may, perhaps, be 

 explained by the diminished outflow of blood in 

 mammals consequent on the greatly reduced size of 

 the ova. According to Wickmann, the pigments 

 formed in this way within the ovary are shed into 

 the oviduct, and mingled with the materials of the 

 shell in its uterine portion. If his observations are 

 correct, they perhaps help to explain the facts noticed 

 by Professor Newton {pp. cit), that when a bird lays 

 only two eggs, it not infrequently happens that all the 

 available pigment is deposited on one, while the other 

 maybecolourless. Professor Newton gives the Golden 



