282 Rogciioration of the ISkin of the Frog 



cell hy tlic 1)]()()(1 oi' lyiiiph. An iiii])i,i;iii('iit(M] cell is surrounded by the 

 same iiuti'it'iit as a ])i,ii'!neiit producing- one; yet the latter only forms 

 pigment. 'I'lie i)igment forming epithelium may be transplanted to a 

 jilacc wlicrc fornu'i'Iy iiiipigmented cells were sitiuited, aiul it will con- 

 tinue to |)roduce pigment, though the blood su])])ly must remain the 

 same. '^I'he production of pigment by these cells can in no way be com- 

 pared to tlie formation of ])igment in connective-tissue cells wliich are 

 in contact with extravasated blood. 



The evidence furnished by these studies against l^^hrmann's hypothesis 

 of the nu'sol)]astic origin of ejudermal chi'omatophores may be summar- 

 ized as follows: 



(1) We find no indications of a migration of pigmented or pigment- 

 producing cells of any kind from the dermis into the epidermis. 



(2) Chromatophores were observed to multiply by mitosis in the epi- 

 dermis, and they are found regularly in regenerating epithelium long 

 before any dermal tissue lias regenerated in the space below. 



SUMMARY. 



1. Solutions of pilocarpine, atropine, and alcohol in wliich the ani- 

 mals liviMJ constantly had little influence on regeneration. 



2. The rapid movement of epithelium over the wound soon after cut- 

 ting the skin is not due to cell proliferations. It is more probable that 

 a tension, either previously existing or called into play by the wound, is 

 the cause. 



3. Cells divide both by mitosis and by amitosis in the regenerating 

 epithelium. In regenerating frog epidermis mitoses are found in higher 

 layers of cells than in guinea-pig skin. 



4. Epithelial cells move in all directions into the sub-epithelial coagu- 

 lum, and they may break through fibers of fibrin. 



5. If the wound is large, the sub-epithelial clot may remain imper- 

 fectly organized, and some connective-tissue cells may degenerate. There 

 is often very little regeneration of connective tissue below the wound as 

 late as three weeks after the operation. 



G. The chromatophores in the epidermis of frog skin behave in regen- 

 eration as ordinary epithelial cells and not as the chromatophores of the 

 cutis. 'I'he former regenerate rai)idly and the latter very slowly. Dur- 

 ing regeneration, epithelial chro!nato])hores may be found in the coagu- 

 lum uiulerneath the e])idermis. 



7. There is no evidence of an ingrowtli of chromatophores into the 

 epidermis rroin the ciiiis, and the t'))ithelium of the regenerating patch 



