Influence uf Physical Agents on Development of the Frog. 241 
but supported with aérated 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 ailowed 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. 
_ The metamorphosis of the tadpole, when at its full growth, requires 
about fourteen days to bring it to the condition of a frog. About 
the termination of that period, the diminution of the body is so great, 
and also the absorption of the expanded caudal extremity is such, as 
to diminish cutaneous respiration. Respiration by the lungs becomes 
absolutely necessary to prevent the animal from becoming asphyx- 
iated, which would be the case if it remained in the water—requiring 
then not an aquatic, but an atmospheric medium of respiration. It 
may be observed that after the tail is partially absorbed, leaving only 
a portion of the solid part, the asphyxiated state has commenced ; 
the little animal, with open mouth, gasps for breath: but if removed 
into atmospheric air, the mouth is directly closed, and respiration is 
effected through the nostrils with perfect freedom ; the animal is re- 
stored directly, jumps about and is lively. 
3. On the Influence of the absence of Light on the. Tadpole and on 
the Frog. 
This time I commenced my experiments in three rock cellars, 
formerly only in one. 
