CAROTINOIDS IN THE VERTEBRATES 127 



of the true relationship of the corpus lutcum and egg yolk pigments 

 to each other and to other similar pigments in plants and animals. 

 At any rate, much of the subsequent confusion of different pigments 

 niiiilit, perhaps, have been avoided. 



Kiihnc (1878), however, was forced to conclude that the corpus 

 luteum and egg yolk pigments were not identical, after examining 

 carefully their spcctroscopic absorption properties. No further study 

 appears to have been made of the corpus luteum pigment until Escher 

 (1913) definitely established its identity with carotin. 



Before referring to other mammalian carotinoids it may be well to 

 point out that we have definite proof that carotin is the corpus luteum 

 pigment only in the case of cows and sheep, from which Escher 

 obtained his material for study. Pigmented tissue appears on the 

 human ovary, also, but there is no evidence that the pigment is exclu- 

 sively carotin. On the contrary the inference which may be drawn 

 from observations regarding the character of the chromolipoids in 

 other parts of the human body is that both carotin and xanthophylls 

 probably appear in the human corpus luteum. Still less is known 

 regarding the pigment in the corpus luteum of other mammals. In 

 the horse it is probably carotin, since this pigment appears in the 

 blood of that animal. Carotinoids arc not present at all in the so- 

 called yellow bodies on the ovaries of swine, as pointed out by van 

 den Bergh, Muller and Broekmeyer (1920). The writer 1 succeeded in 

 extracting a small amount of yellow coloring matter from swine 

 ovaries when a sufficient number were extracted, but all attempts to 

 identify the pigment as carotinoid resulted in failure. 



Blood serum. Although the carotinoid of the corpus luteum of the 

 cow was the first mammalian chromolipoid to be isolated in crystal- 

 line form, the coloring matter in the blood serum of cattle was prob- 

 ably the first to attract attention. Krukenberg (1885a), who deserves 

 credit for the first extensive study of the pigment, mentions the much 

 earlier attempts of Samson (1835), Denis (1838) and Schmidt (1865) 

 to determine its nature. It is true that Thudichum (1869) stated 

 that the yellow pigment of blood serum belonged to his group of 

 luteins, but he did not trouble to mention the animals in which he 

 had found it, or how he had isolated the pigment. As a matter of fact 

 Krukenberg (1885a) found it to be rather difficult to separate the 

 pigment of cattle serum from the other blood constituents; direct 

 extraction with all the known fat solvents failed completely, and 



1 Unpublished observations. 



