MELANOPHORES OF RANA 245 



of which anastomose. Within the 'basket' he one or more xan- 

 tholeucophores. The color changes produced are so well known 

 as to need no further description. 



The exact path of movement of the indi\'idual pigment gran- 

 ules during expansion and contraction is difficult to determine, 

 owing to the thickness of the frog's web or the pieces of tissue 

 in the plasma cultures. No evidence has been obtained, how- 

 ever, that tends to demonstrate the presence of any other type 

 of movement than that observed in the corresponding cells of 

 tadpoles. On the contrary, the two types of movement are very 

 similar. 



In cross sections through well fixed frog skin, the fine histo- 

 logical details stand out with remarkable clearness.^ One point 

 of difference between the larval and adult melanophores is very 

 evident. Whereas, in the former, the cytoplasm of the cell is 

 almost always visible in fixed material, in the latter, the amount 

 of pigment is so much greater that the cytoplasm may be seen 

 only in those cells which are almost, but not quite, completely 

 contracted. In both the fully expanded and the completely 

 contracted phases the pigment granules fill the cytoplasm so 

 completely as to obscure it. As in the larval melanophores, the 

 pigment granules themselves are not arranged in any definite 

 order, so far as could be determined. 



The completely contracted melanophore of the adult appears 

 as a rounded or oval mass of pigment and, like the larval, occu- 

 pies a fixed cavity. The shapes of the two types of cavities are, 

 however, entirely different. The cavity occupied by the adult 

 cell consists of a central reservoir but slightly larger than the 

 contracted cell, from which tubular branches, one for each process 

 of the expanded cell, extend into the surrounding tissues (fig. 3). 

 As noted in a previous paper, these cavities seem to have an 

 endothelial lining. The lumen of the branches remains patent 

 for varying distances from the cell body, but the tips are usually 

 collapsed, either by intercellular pressure alone, or, as seems 



* A Ziess apochromatic 2 mm. X. A. 1.40, immersion objective and 3, 4 and 12 

 compensating oculars were usetl in examining the histological details described 

 in this paper. 



