40 



Mr. H. Onslow. 



was totally unlike that of any tube in the former experiments. A heavy 

 charcoal-black ring, \ inch wide, had formed in the upper portion of the 

 fluid, while the lower portion remained unaltered. When the tube was 

 shaken and allowed to incubate again, a fresh black ring formed on the 

 surface of the fluid where it was in contact with the atmospheric oxygen. 

 Occasionally in its earlier stages this black fluid had a reddish purple tinge, 

 and a variable amount of black precipitate was always deposited after the 

 lapse of some days. The results were the same if the tubes were kept at 

 room temperature, but the darkening took place less rapidly. This seemed 

 to be a true oxidation due to a peroxidase from the skin extracts in the 

 presence of tyrosine and hydrogen peroxide. This reaction was accordingly 

 always used as a method of testing for an inhibitor, in the manner afterwards 

 described. Each experiment was repeated a number of times, and in every 

 case with the necessary controls. 



(a) Material — 



The skins employed in these experiments were taken from a number of 

 rabbits of the age of from two to four days, and during the course of the 

 experiments over 200 young rabbits were employed. It was impossible to 

 extract any ferment from older rabbits, owing probably to the increased 

 toughness of their skins. Black rabbits were always used for the purpose of 

 obtaining an active tyrosinase with which to test for the presence of an 

 inhibitor. The nature of the tyrosinase in agouti, chocolate, orange, yellow, 

 and blue rabbits, as well as in black mice, was also investigated. Since 

 there is no true dominant white variety of rabbit, recourse was had to the 

 type known to breeders as " English." This is a white animal with black 

 eyes, patches of colour on the ears, face, and flanks, and a continuous line 

 down the middle of the bach. This pattern is said by Hurst* and Castlef 

 to be dominant to self colour, a fact which was fully confirmed by breeding- 

 experiments carried out in connection with this research. 



(b) Methods— 



About eight black rabbits, preferably of the same litter, from two to four 



days old, were anaesthetised, killed, then carefully skinned by making an 



incision down the back, and the skins freed from subcutaneous tissue. The 



greatest care must be taken to remove completely any small blood-vessels 



that may be seen, otherwise the resulting fluid will be tinged with a trace 



of haemoglobin which might vitiate the results. The skins were next 



thoroughly rinsed with water, dried with a cloth, weighed, and finely minced 



in a small mincing machine. The pulp was put into a mortar with about 



* Hurst, 'Report Confer, on Genetics, Roy. Hort. Soc. London,' p. 114 (1906). 

 t Castle and Hadley, ' Amer. Nat.,' vol. 49, p. 23 (January, 1915). 



