88 ANN HAVEN MORGAN 



and heat were unlike in effect and that the photoreceptors were 

 much more easily excited than the receptors for heat. A slight 

 difference in light, on the other hand, made no impression on 

 frogs with which Torelle ('03) worked. They swam up and down 

 in the jars regardless of adjustments of light and dark. 



Frogs are stereotropic in temperatures between 10° C. and J^°C. 

 When Torelle placed frogs in water cooled to 10°C. or below, they 

 flattened their bodies against the bottom or crept under rocks 

 placed on the floor of the aquarium. 



Effect of temperature on responses to electricity. An electric 

 current which produced tetanic movements on a warm frog 

 showed retardation when the frog was cooled (Kunde, '60). 



METHODS 



The experiments which follow were performed upon green 

 frogs (Rana clamitans) and leopard frogs (Rana pipiens) in a 

 laboratory the temperature of which varied between 18°C. and 

 23°C. The work was done between October and January upon 

 animals which were kept in a basement tank and brought into 

 the laboratory at least two days before they were used for 

 experimentation . 



For all except one experiment, the front part of the head was 

 removed by a single transverse cut made just in front of the 

 eardrums. Through the lower jaw thus left intact a loop of 

 silk was drawn, and by this the frog was suspended, thus avoiding 

 the irritation caused by the repeated use of a metal hook. 



Frogs were hung from an extension bar, attached to a standard ; 

 the bar could be easily raised and lowered. They were com- 

 pletely immersed in a bath of water at the beginning of each 

 experiment, and at certain intervals during treatment in order 

 to keep the temperature normal, the skin moist, and free from 

 particles of dust. At the beginning of an experiment the tem- 

 perature of the room, bath water and frog were taken, the latter 

 being secured by putting a thermometer through the mouth 

 and down into the stomacl^. Records of these temperatures 

 have been given with each experiment recorded in this paper. 

 The experimental frogs were easily kept in good condition and 

 usuallv li'\''ed from four to five weeks. 



