72 University of California Publications in Zoology [Vol. 11 



The darkening toward the surface proceeds more slowly also 

 showing more of the red and mahogany tints before reaching 

 black. The solutions become as dark as the others if left long 

 enough, and when shaken show the same slight difference between 

 the yolk and the liver and albumen. Liver and albumen tubes 

 are often the same shade, but if there is a difference, the albumen 

 tube is usually darker. The control tube acts more like pepsin 

 than panereatin, retaining its pink color for some time as the 

 pepsin filtrates do. 



The more rapid formation of color in the panereatin tubes 

 as compared with the pepsin or control tubes is, I think, due in 

 part to the fact that the filtrates contain products of digestion, 

 among which are probably some which serve as chromogens. 

 Tyrosin is present among the cleavage products of most foods; 

 and there may be other chromogens present. Their presence 

 would account for the fact that the test tubes containing pan- 

 ereatin filtrates color more quickly than the control tubes do. 

 The tubes containing pepsin filtrates, on the other hand, prob- 

 ably contain enough pepsin to slow down the reaction, since we 

 found that pepsin alone inhibits the reaction markedly. It is 

 also possible that they contain less chromogen than the panereatin 

 filtrates. Probably both factors operate to produce the result, a 

 color reaction which generally runs parallel with that of the 

 control tubes. 



Gessard (1901) obtained a similar result when he found that 

 forty drops of blood serum from a calf retarded the tyrosinase 

 reaction nine daj's, while fifty drops of water retarded it for a 

 month. It is highly probable that some chromogen was present 

 in the blood serum which would at least contribute more toward 

 pigment formation than would any elements contained in an 

 equal amount of water. 



Though the results of the experiments are not altogether con- 

 stant, the variability in the reaction is probably to be accounted 

 for partly bj' the fact that different eggs and livers were used 

 in different experiments, partly by the fact that the foods may 

 have reached different stages of digestion in the different sets of 

 experiments, and perhaps by variability of other and unknown 

 factors. In spite of these differences, however, the experiments 



