SKIN TRANSPLANTATION IN FROG TADPOLES 363 



would cause a change in the amount of the specific substances 

 produced, and this, in turn, would cause expansion or contrac- 

 tion of the melanophores according to conditions. Redfield ('18) 

 demonstrated that in the horned toad adrenin is the specific 

 substance for the pigment cells. The effect of light is not so 

 clearly explained by this assumption. However, it is commonly 

 true that light acts on animals as a stimulant, causing an increase 

 in metabolism, and that the absence of light acts like a narcotic. 

 Assuming this for the frog tadpole, which is known to be photo- 

 kinetic (Cole, '17; Obreshkove, '20), the hypothesis is supported 

 by the facts of the experiments. 



2. Description of the healing process 



The removal of integument from any part of the tadpole's body 

 initiates a stimulus which forces the epidermal cells surrounding 

 the wound to migrate over it centripetally until a new covering 

 is formed. This layer is at first only one cell thick, but further 

 migration from the edges deepens it to several cells. As they 

 move over the wound they carry along epidermal melanophores. 

 Further increase in thickness of the new layer results from mi- 

 toses, which continue at a rate slightly above normal, until the 

 original condition is closely approximated. 



The rapid movement of epithelial cells over a fresh wound was 

 first observed in the cornea of the frog by Peters ('85) and later 

 in the salamander by Barfurth ('91) and again in the frog by 

 Loeb ('98) and by Loeb and Strong ('04), who thought that it 

 was caused by a tension ''either previously existing or called into 

 play by the wound." Such migration of epithelial cells has been 

 seen in many different animals, and in tissue cultures it is a com- 

 mon phenomenon. In commenting on the reaction in actinians. 

 Rand ('15) says: 



In general an epithelium will not tolerate a free edge. When such 

 an edge arises, accidentally or otherwise, the epithelium extends until, 

 if possible, the free edge meets and unites with some other portion of 

 the same layer or with another epithelium. The essential function of 

 an epithelium is to cover a surface continuously. 



