An exceedingly interesting observation has been recorded by Stevenson (10) working in the laboratory of F. G. Hopkins. She states that an impure preparation of carotin, which did not behave like a source of fat-soluble A when fed as such, acquired the properties of this dietary factor when it was dissolved in palm-kernel oil previously tested and found ineffective for the stimulation of growth. This observation deserves the most careful attention, for if substantiated, it would seem to indicate that fat-soluble A may not be absorbable and utilizable unless it is carried by at least small amounts of fats. She reports also that butter fat can be decolorized of its yellow pigment by absorbing the carotin with charcoal and without impairing in any way its value as a source of fat-soluble A. It will be seen, therefore, that the evidence up to this time is decidedly conflicting on the point as to the relation between yellow pigment and content of fat-soluble A.

Palmer and Kennedy (11) have, however, published an excellent paper on this subject which finally disposes of the question, for they prove conclusively that there is no correlation between the pigmentation of certain foods and their value as a source of fat-soluble A. A brief history of Palmer's studies should precede the discussion. Palmer first took issue with Steenbock on the basis of the former's observation that cottonseed oil, when freed from its resinous substance, possesses a fine golden yellow color and is rich in carotinoids, but does not contain demonstrable amounts of fat-soluble A (12). He also showed that the blood of certain species of animals, as sheep, swine, dog, cat, rabbit, and guinea pig, is free from carotinoids. This would seem, if the vitamin were one of the pigments, to preclude the entrance of the substance into the tissues of these animals. Dolly and Guthrie (13) found adipose tissues and nerve cells of these animals to be free from carotinoid pigments.