535 



cutaneous surface. The gill-opening for the passage of water is 

 very apparent on the left side, but there is none on the right. 



2. On the Influence of Light on the Tadpole. 



The experiments of Dr. Edwards indicate that a decided influence 

 is exerted by light upon the metamorphosis of Batrachians, since, 

 according to his statement, when tadpoles which had arrived at 

 nearly their full growth were secluded from the influence of light, 

 but supported with aerated water and food, they attained an extra- 

 ordinary size, without undergoing any metamorphosis. The follow- 

 ing is Dr. Edwards' s experiment : " I procured a tin box, divided 

 into twelve compartments, each of which was numbered and pierced 

 with holes so that the water might readily pass through the box. 

 A tadpole (which had been previously weighed) was put into each 

 compartment, and the box was then placed in the River Seine, some 

 feet below the surface. A large number were at the same time put 

 into an earthen-ware vessel, containing about four gallons of Seine 

 water, which was changed every day ; these tadpoles were at liberty 

 to rise to the surface and respire air, and they soon went through 

 their metamorphosis. Of the twelve placed in the box under water, 

 ten preserved their form without any progress in their transforma- 

 tion, although some had doubled or trebled their weight. It should 

 be observed that at the time when the experiment was begun, the 

 tadpoles had attained the size at which the change is about to take 

 place. Two only were transformed, and these very much later than 

 those which, in the earthen vessel, had the liberty of respiration in 

 air." Dr. Edwards concludes that the presence of solar light favours 

 the development of form. 



The situation in which Dr. Edwards placed the tadpoles, " some 

 feet below the surface of the river " in his experiment, would inevi- 

 tably prove unsuccessful in the full development of the frog. I have 

 always found the transformation, both of the triton and of the frog, 

 equal in the same temperature, both in the light and in the absence 

 of light, if placed in shallow water ; but during their metamorphosis 

 they must be allowed to rise to the surface of the water to obtain 

 air, or they become asphyxiated. I therefore placed stones in 

 the vessel, and allowed them to leave the water for the purpose of 

 atmospheric respiration. 



