1914] Holmes: Melanophores of the Frog 169 



The movements of the smaller melanophores are very much like 

 those of an Amoeba. Pseiidopods are thrust out and retracted, and 

 often broad, thin sheets of protoplasm may be extended from the cell. 

 In several cases a considerable amount of migration was observed ; 

 in one instance this amounted to over ten times the diameter of the 

 cell. When first formed, the pseudopods appear as a thickening- of 

 clear ectoplasm Mdiich may, however, be very quickly invaded by 

 endoplasm containing black granules of pigment. In many cases the 

 pseudopod may reach a considerable size before it comes to contain 

 any pigment granules, and often broad sheets of protoplasm may be 

 thrust out, in which pigment is almost entirely absent. The forms 

 assumed by the melanophores are very varied. In the larger pigment 

 cells the processes are generally more numerous, relatively longer, and 

 more branched. The pseudopods of the larger cells may be several 

 times the diameter of the body of the cell, which they seldom are in 

 the smaller ones. They may contain pigment nearly to the extreme 

 tip, or they may be almost entirely devoid of it, different processes of 

 the same cell presenting great differences in this respect. 



The pigment cells of the adult frog differ therefore from those 

 of the young larvae in that they frequently possess transparent pro- 

 cesses. To a certain degree pigment may flow back and forth within 

 these processes. But the largest factor in determining the changes 

 in pigment distribution, especially in the smaller melanophores, is 

 the change in the outline of the cells themselves. 



The behavior of advancing pseudopods indicates that they are not 

 pushed out, but pulled out. The Pseudopods that are being extended 

 are not blunt and evenly rounded as they would be if pushed out by 

 the pressure of the endoplasm. When the pseudopod is broad the 

 ectoplasm at the advancing extremity is commonly in the form of 

 a broad, thin sheet with very fine processes of transparent protoplasm. 

 These processes are closely applied to the substratum to which they 

 adhere. This thin, transparent region of protoplasm is the chief seat 

 of locomotor activity. The part lying behind it is apparently pulled 

 along by the contractility of the ectoplasm. The end of an advancing 

 pseudopod is commonlj^ broader and thinner than the region lying 

 behind it. A certain tendency to spreading at the extreme end usually 

 characterizes also the very long and narrow processes that sometimes 

 appear. The proximal part may then be nearly cylindrical and almost 

 uniform in diameter and gives every appearance of having been 

 pulled out. When the pseudopod is retracting, the tip is commonly 



