240 DAVENPORT HOOKER 



tissue melanophores, inasmuch as they are not strictly corial in 

 the tadpole. Exposure to light causes the melanophores of frog 

 larvae to expand, while confinement in darkness causes them to 

 contract. Both processes are completed in a little less than an 

 hour, so that the changes in form of the cell may be observed in 

 detail and fully controlled. 



In the living tadpole, the connective tissue melanophores ap- 

 pear as small, round dots when fully contracted. It is impossi- 

 ble, even with high magnification, to see the cytoplasm and only 

 under the most favorable conditions is a faintly marked, rounded 

 nucleus to be seen lying to one side of the pigment mass. If 

 such cells be exposed to bright light, either from above or from 

 below, they begin to expand. In the living specimen, expan- 

 sion and contraction of the cell is indicated by the movement 

 of the pigment granules alone, as the cytoplasm is invisible. 

 The pigment begins to stream away in every direction from the 

 previously rounded mass, at first as a fine line of granules, then 

 as a sheet of pigment which increases in breadth until the fully 

 expanded condition is reached. 



During the process of expansion of the melanophore, the pig- 

 ment granules do not move out from the central mass along 

 radiating lines nor do they follow any definite pathway, but 

 move about, now forward, now sideways, now backward, in 

 exactly the same manner as small particles of lampblack sus- 

 pended in a drop of water which is slowly rolling down an in- 

 clined plane. Further, it is noticeable that, in successive expan- 

 sions of a single cell, its processes are never exactly alike, owing 

 to the fact that it does not always expand at exactly the same 

 rate on all sides. 



The completely expanded melanophores of the tadpole are 

 thin, light brown sheets of pigment with short, broad processes 

 of uneciual length. A little to one side of the center of the cell 

 is a small area somewhat lighter in color than the rest, although 

 the pigment granules are apparently as thickly distributed here 

 as elsewhere. This is the highly refractive nucleus lying in the 

 cell body. No such pigmentless area as the 'Attraktionssphiire' 

 described by Ballowitz, Solger and others was observed. 



