524 THE THIRD HUXLEY LECTURE 



ference of arrangement of its pigmentary constituents. Where the mustard 

 had rested, the pigment appeared as a dehcate black network among the tissues, 

 causing an extremely dark appearance ; whereas in the rest of the web it was 

 gathered into small round spots, which interfered little with the pallor of the 

 other structures. I at once saw that I had before me direct ocular evidence 

 of an effect of the irritant upon the tissues. The circumstance that I had applied 

 the mustard to one spot only of the web, had revealed what had escaped the 

 notice of the many previous observers who had studied the circulation in the 

 frog. I afterwards learned that changes of colour due to pigmentary variation 

 had been observed in Germany in the green tree-frog by Von Wittich, who had 

 attributed them to contractions and relaxations of chromatophorous cells, 

 more or less analogous to what is seen on a large scale, visible to the naked eye, 

 in the skin of cephalopods. Very different were the real pigmentary functions 

 in the frog. The colouring matter, which was in the form of granules of extreme 

 minuteness, was contained in cells with offsets that rapidly broke up into rami- 

 fications of exquisite delicacy, anastomosing freely with each other and with 

 those of other branches and of neighbouring cells, only visible when the frog 

 was at the darkest, when they appeared, under the highest magnifying power 

 at my disposal (a fine — ^^ Powell and Lealand's), as fine homogeneous black 

 lines, in which the closely packed granules were not individually discernible. 

 Under these circumstances the bodies of the cells and their principal offsets were 

 so cleared of pigment as to be almost colourless, so that it was difficult to define 

 their contour. 



On the other hand, when the animal was at the palest, the pigment-granules 

 were massed together into a circular disc, which did not occupy the whole of 

 the body of the cell, being apparently grouped round its nucleus, while the offsets 

 and their ramifications were quite colourless. Any intermediate degree between 

 these extremes of complete diffusion and perfect concentration of the pig- 

 ment-granules might occur, with corresponding differences in the tint of the 

 animal. 



Camera-lucida sketching here stood me in good stead. I doubt if any one 

 would have credited my description had I not been able to support it by such 

 evidence. For here was a function entirely new to physiologists. In muscular 

 contraction the entire mass of the cell shrinks, and in ciliary action, the only 

 other visible form of motion then known to occur in animal tissue, the part 

 concerned moves as a whole, so far as we are able to observe it ; in the 

 pigmentary changes the form of the cell remained unaltered, but one of 

 its constituent materials was seen to be transferred from place to place 

 among the rest. But drawings made with the camera of a cell in sue- 



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