280 Eegeneration of the Skin of the Frog 



The pigmentation of frog skin differs considerably from that of the 

 gninea-pig. In the frog cutis there is usually a well-marked layer of 

 chromatophores, which are generally much larger and frequently more 

 branched than the epidermal chromatophores, whereas the guinea-pig 

 cutis has no well-developed chromatophores and its pigment is distrib- 

 uted irregularly in masses or clumps. The dermal chromatophores of 

 the frog are separated from the epidermis by considerable connective 

 tissue, and the epidermal chromatophores are usually situated higher up 

 in the epidermis than is the case with the guinea-pig. 



L. Loeb, 98, distinguished four stages in the development of pigmenta- 

 tion in the regenerating black skin of the guinea-pig. These were not 

 observed in the regenerating frog skin. 



x\s in the case of the guinea-pi^, we find no evidence of an immigra- 

 tion of dermal chromatophores into the epidermis of the regenerating 

 frog skin. 



The epithelium, which moves over the wound soon after the operation, 

 carries chromatophores and ordinary pigmented epithelial cells. These 

 chromatophores are usually found to be without processes. During the 

 first two weeks similar chromatophores are frequently observed in the 

 regenerating epithelium. They may still be found during the third week, 

 especially in the central part of the regenerated epithelium. Under 

 these conditions they may appear as ordinary pigmented epithelial cells ; 

 they carry, however, more pigment than the latter. Kromayer has 

 also observed chromatophores without processes near the margins of 

 wounds in amphibia. 



The number of well-developed chromatophores with large processes 

 increases gradually in the regenerating epithelium, and they become 

 especially numerous near the margins of the regenerating area. Chroma- 

 tophores without processes were found, however, even at later periods, in 

 the hypertrophied epithelium where cell-degeneration occurred. 



Chromatophores divide mitoticajly during regeneration in frog skin. 

 Two chromatophores were found in mitosis at places ivhere ordinary 

 epithelial cells were also dividing mitotically, one at fourteen and the 

 other at nineteen days after the operation. One showed processes but 

 the other had none. 



In regenerating epithelium, the chromatophores are not arranged in 

 as regular a manner as in ordinar}!- epithelium. During the first two 

 weeks many chromatophores of the epithelium covering the wound and 

 occasionally also of the adjoining epithelium, are carried into the upper 

 part of the epidermis and are frequently cast off. Sometimes the chro- 

 matophores are pushed into the lower layer of the epithelium, and even 



