CAROTINOIDS IN Till: VERTEBRATES 139 



The probability of a definite chemical relation between the egg yolk 

 pigment and plant cnrotinoids was pointed out for the first time by 

 Schunok (1903) who found that the spectrum of the alcoholic solution 

 was identical with one of the xanthophyll group of pigments which 

 lie isolated from a number of flowers. Schunck's work is described in 

 Chapter II. It was pointed out there that Schunck's method for sep- 

 arating the xanthophylls is not exact. In all probability, however, 

 the L xanthophyll which he described, and which showed the same 

 spectroscopic properties as the egg yolk lipochrome, corresponds best 

 with Tswett's a xanthophyll. This appears to be the xanthophyll 

 which is present in the chloroplastids in greatest amount. 



The definite chemical identification of the egg yolk pigment of hen's 

 eggs as xanthophyll soon followed, when Willstatter and Escher (1912) 

 isolated the crystalline pigment and showed that it corresponds in 

 all its chemical and physical properties, except its melting point, with 

 the crystalline xanthophyll of green plants. The failure of the egg 

 yolk xanthophyll to correspond in its melting point with the plant 

 xanthophyll of Willstatter and Mieg (1907) has never been explained. 

 Serono (1912) has criticized Willstatter and Escher's work severely, 

 and expressed the opinion that the product which they isolated was 

 not a carotinoid at all, but a cholesterol ester of oleic acid. He shows 

 how the elementary composition of such an ester corresponds even 

 more closely with the analyses of the egg yolk xanthophyll than the 

 latter does with plant xanthophyll, and advances the belief that this 

 explains the high melting point found by Willstatter and Escher for 

 the egg yolk pigment. Serono's explanation of the high melting point 

 of the egg yolk xanthophyll falls to the ground, however, in the light 

 of the studies of the writer (1915) which showed that the egg yolk 

 pigment is not only chemically related to plant xanthophyll but is 

 biologically derived from it. Whether the hen's body modifies slightly 

 the plant pigment, thus giving it a different melting point from its 

 precursor, or whether the hen selects one of the several plant xantho- 

 phylls differing in melting point from the mixed product obtained by 

 Willstatter and Mieg from nettle leaves, or whether the difference is 

 to be explained on other grounds cannot be decided definitely at the 

 present time. 



Xanthophyll is not the only carotinoid in the yolk of hen's eggs. 

 In their isolation of the crystalline pigment Willstiittcr and Escher 

 noticed the presence of a small amount of pigment with the solubility 

 relations of carotin. The writer (1915) was able to confirm this in his 



