CHEMICAL RELATIONS BETWEEN CAROTINOIDS 177 



cholesterol ester constitution for the egg yolk pigment, it must be 

 admitted, nevertheless, that if it were not for the complete corre- 

 spondence of the general chemical properties of the egg yolk pigment 

 with plant xanthophyll, it would not be possible to decide on chemi- 

 cal grounds, from the evidence submitted, that the two substances 

 are identical or even isomers. 



Corpus lut< nni ctirotin. It was pointed out in a previous chapter 

 that the pigment in the corpus luteum tissue on the ovaries of the cow 

 was one of the first to attract the attention of those interested in dis- 

 covering the nature of animal pigments. It was certainly the first 

 animal carotinoid to be obtained in crystalline form if one is to accept 

 the early work of Piccolo and Lieben (1866) and Holm (1867). 

 Although a number of later workers described the chemical prop- 

 erties of the pigment, as was shown in Chapter IV, Willstatter and 

 Escher (1912) are to be credited with first stating the exact chemical 

 relationship of the corpus luteum pigment to plant carotin. Their 

 work was not the first to show a connection between animal coloring 

 matter and plant carotin; nor were they the first to use the name 

 carotin for an animal pigment. It will be recalled that -Zopf used 

 the name carotin in the form of "eucarotin," "di-carotin," "mono- 

 carotin," "carotinin," etc., for various animal pigments studied by 

 him. He certainly recognized the relation between the vegetable and 

 animal "carotins," to which he gave the various designations men- 

 tioned. It is apparent, however, that the name itself w^as a collective 

 name in Zopf's mind and that both carotin and xanthophylls, in the 

 sense we now know them, were included. For example, Gerlach 

 (1892) working under Zopf's direction, refers to the egg yolk pigment 

 as di-carotin. On the other hand Physalix (1894) actually had 

 Arnaud's carrot carotin in mind, as his paper shows, when he assigned 

 the name carotin to the pigment which he isolated from the red tegu- 

 ment of the bug Pyrrhocoris apterus. The insect carotin was not, 

 however, analyzed. 



It was no doubt the development of the Kraus method for separat- 

 ing the plant carotinoids into specific carotin and xanthophyll groups, 

 in which Tswett and Willstatter played a dominant part, that led 

 Willstatter to examine ci-rfain of the common animal lipochromes by 

 this procedure. The discovery of the xanthophyll character of the 

 egg yolk pigment by this means naturally prompted the study of the 

 corpus luteum pigment, since the observations of the early workers 

 had shown that it could be obtained in crystalline form without dif- 



