73 
tissues of the negro and the lack of its general production in the 
epidermal tissues of the white races under normal conditions. The 
lact, however, that melanotic pigments are formed over limited areas 
in the dermal structures of white-skinned races indicates that the 
same causes of pigmentation may be operative in all races to a greater 
or less degree and serves to emphasize the importance of further 
studies on the mode of action of tyrosinase and its mode of origin in 
animal tissues. 
The occurrence of tyrosinase in the skins of certain pigmented 
vertebrates has been investigated by Miss Florence M. Durham ( 155 ). 
Aqueous extracts of the skins of rabbits, rats, guinea pigs, and chickens 
in the foetal stage of development, containing small amounts of fer- 
rous sulfate, were found to act upon tyrosin with the production of 
pigments similar in tint to those characteristic of the coat of the ani- 
mal. Thus black pigments were formed with extracts of the skins 
of black-pigmented animals, and yellowish pigments with those of 
the skins of animals containing orange-colored pigments, and with 
extracts of the skins of white or albino animals no pigments were 
formed by the action of an extract of the skin on tyrosin. The tyrosin- 
ase present in the skins of animals was found to act most rapidly at 
37° C. It is destroyed by boiling and does not act in the cold. From 
one to ten days were required for the production of the pigment in 
vitro. Another peculiarity of the tyrosinase contained in the skins 
of animals is that it only acts upon tyrosin in the presence of small 
amounts of iron. 
Phisalix ( 321 ) has obtained tyrosinase from the skin of the green 
fro^ md Gessard ( 188 ) has found it in the skins of frogs of other spe- 
cie^, ju JtL as the frog rousse and the common toad. More recently 
Phisalix ( 322 ) has investigated the color changes occurring in the 
larvae of the cockroach (Phyllodromia germanica). Immediately 
after hatching from the egg the integument of the larva is soft and 
bright white in color. In the course of a short time, however, it be- 
comes grey, then brown, and finally black, so that three hours after 
hatching the little cockroach is entirely black. According to this 
author this change of color is the result of the action of tyrosinase 
on tyrosin, both of which occur in the embryo at the moment of its 
development ; indeed, it is probable that they are both present in the 
egg in which they are deposited at the time of ovagenesis. 
Still more recently a very valuable contribution to our knowledge 
or melanotic pigments and fermentative melanine formation has been 
made by Von Furth and Jerusalem ( 178 ). These authors have com- 
pared hippomelanin (the black pigment produced in tumors of the 
tmrse) with other natural and artificial melanins, both as to mode 
or formation and general physical and chemical properties. They 
-d hippomelanin to be iron free and are inclined to look upon its 
