MELANOPHORES OF RANA 241 



111 the fully expanded living melanophore, the pigment gran- 

 ules are never at rest, but move about constantly. This fact, 

 together with the absence of any definite channel for their expan- 

 sion, indicates that the intracellular canal system described by 

 Ballowitz ('13) for the teleosts does not exist in the tadpole. 



Inasmuch as the tadpoles must be kept in total darkness to 

 obtain contraction of the melanophores, this process may not 

 be observed as readily as expansion. By examining them at 

 intervals, however, it is evident that contraction is merely a 

 reversal of expansion. 



The arrangement of the pigment granules in the expanded 

 phase of the lar\'al cell ma}^ be seen to advantage in carefully 

 fixed material. They have no definite relation to one another 

 or to the nucleus, but are uniformly distributed throughout the 

 cytoplasm. The nucleus appears here as a dark, rounded area 

 in contradistinction to its light, refractive appearance in the 

 living cell. 



, In cross-sections through expanded melanophores, they appear 

 as thin, elongated masses, tapering gradually toward the two 

 extremities and contain, near the center and toward the deep 

 surface of the cell, the nucleus. The latter is in every respect 

 similar to the nuclei of the surrounding connective tissue cells. 

 The pigment granules, when seen in these sections, again do not 

 lie in radiating lines from the center of the cell to the periphery, 

 but are indiscriminately scattered through the protoplasm, which 

 is visible in the preserved material. Such sections also throw- 

 light on the relation of these melanophores to their environment. 

 They are situated in an extremely thin sheet of connecti\'e tissue 

 cells and fibers which form a series of compartments or spaces 

 (fig. 1) in which the melanophores lie. The walls of these spaces 

 are extremely thin and the cavities themselves have no definite 

 form. Frequently, the cavities overlap at their borders. They 

 are merely connective tissue spaces and, as the melanophores 

 never completely fill them, do not determine the pattern of the 

 expanded cell. 



Fully contracted larval melanophores present the following 

 appearance when seen from above (fig. 2) : the pigment is massed 



