170 University of California Publications in Zoology [Vol. 13 



blunt and the proximal part often becomes irregular in contour. 

 Other tissue cells, especially those of the peritoneal epithelium, fre- 

 quently produce pseudopods M^hich greatly exceed in length and 

 tenuity those which arise from the melanophores, and the method of 

 their formation is clearly the same as that just described. The «'n- 

 largements at the end of the outgrowing nerve fiber, which were 

 observed by Harrison (1910) in living neuroblasts, and which have 

 been found in stained preparations by various writers, have also much 

 the same character. In these cases, as in the extension of sheets of 

 ectodermic epithelium, the hyaline amoeboid protoplasmic tip or 

 border of the advancing cells appears to be the active region which 

 is responsible for the progressive movement. 



Unfavorable conditions cause the pseudopods to be withdrawn and 

 the pigment cell to assume a more rounded contour. The fine pro- 

 cesses of the transparent ectoplasm do not appear, and the cell becomes 

 quiescent. Exposure to a higher temperature tends to produce a con- 

 tracted condition of the cells, but the reaction is not marked short of 

 a temperature which is injurious. 



Several experiments were performed to ascertain if the pigment 

 cells would respond to light. Inasmuch as light has been described 

 as having a blanching effect upon excised pieces of frog's skin it was 

 thought probable that a direct effect on the isolated melanophores 

 might be demonstrable. In all the experiments with light the heat was 

 screened out by passing the light through several inches of cool water. 

 A sharply defined beam of strong light was thrown on one part of an 

 active melanophore, but even continued exposure failed to show that 

 the reactions of the exposed part w^ere in any wa}^ different from 

 those of the shaded region of the cell. Pigment cells would remain 

 expanded for over an hour in the strong glare of direct sunlight which 

 the unprotected eye could scarcely endure. One active melanophore 

 was alternately exposed to the intense light of an arc lamp and to 

 darkness for the greater part of a da}-, and was sketched at regular 

 intervals of twenty minutes. On the average the sketches after ex- 

 posure to light did not show that there was any greater tendency to 

 contract in the light than in the dark. Since the cell usually changed 

 considerably in outline between successive sketches, if there were any 

 marked tendency to contract or to expand under the influence of light 

 it would probably have become manifest. The light used in this 

 experiment was so strong that the object could be observed only 

 through smoked glass held over the eye-piece of the microscope. Other 



