50 COLOR CHANGES IN ANIMALS 



stances it was sterilized by heat before it was extracted, 

 but in the beginning this step was avoided. Whether 

 the paste was sterilized or not, its extraction was always 

 carried on at the low temperatures of an ordinary ice 

 refrigerator. After the paste had stood some fifteen 

 hours or so, it was mixed with its own volume of sterile 

 seawater, and the thick liquid that resulted was set 

 aside to allow the oil to rise to the top. In this way 

 there was collected a water-and-oil emulsion which after 

 having been roughly filtered through sterile cheesecloth 

 was vigorously agitated and injected subcutaneously in 

 appropriate amount into a dark dogfish. Very soon 

 after the injection had been made there commonly ap- 

 peared on the skin of the dogfish and a little in front of 

 the point of insertion a few small white spots which 

 however soon disappeared. As these spots appeared 

 when small amounts of indifferent fluids were injected 

 as checks they were regarded as of purely operative 

 origin. In from one to two days after the injection 

 relatively large pale areas made their appearance in the 

 skin immediately over the region into which the fin 

 extract had been introduced (Fig. 36). These large 

 areas were very persistent and, as could be shown under 

 a low power of the microscope, they were produced by 

 the concentration of melanophore pigment. That the 

 pale skin included in these spots was essentially normal 

 was demonstrated by the injection of pituitrin into a 

 fish with such a spot. Shortly after an injection of this 

 reagent had been made, particularly if the region of 

 injection was close to the pale spot, it disappeared by 

 the darkening of its melanophores only to return after 

 a few hours as the effect of the pituitrin wore off. 



These large pale spots were not produced by injec- 

 tions of seawater, oil, oil extracts of dark fins or of 

 muscle, seawater extracts of pale fins, or defibrinated 



