564 
immediately in equal quantities, these are placed in liquids containing 
potassium or in which different quantities of uranium-salt had been 
dissolved. There is practically no potassium found in the Utrecht water, 
thus, as much care as possible was taken, to bring up the uranium- 
tadpoles, with food containing no potassium, while the tadpoles that 
had been put in the liquid containing potassium, got as much ordinary 
(animal) food, as was possible. Rice, boiled in distilled water was 
given as food without potassium. In the first year the uranium-tad poles 
were brought up in glass-bowls; to prevent the dissolving of potassium 
from the glass, the tadpoles were brought up in quartz bowls in 
the second and third years of the experiments. 
I. A first series of experiments consisted in adding to the water 
a certain quantity of uranium-nitrate UO,(NO,),. 
The potassium which was present in the eggs was compensated by 
giving 44 mgr. uranium-nitrate pro liter; moreover another 12,5, 
25 mer. (altogether 17 and 29} mgr.) and 50 mgr. pro liter. 
At the same time as the tadpoles in these liquids, others were 
brought up in ordinary water, with piscidine*) and rice. 
The eggs were laid on April 14", On May 29* the piscidine- 
tadpoles are long 11—12 m.m.’) and have hind-limbs; on June 6'% 
there are tadpoles of 14.2 m.m., which have hind- and front limbs; 
then metamorphosis and tailreduction regularly follow. On June 26" 
only a few tadpoles of 8—12 m.m. remain, these are not yet meta- 
morphosed. 
The rice-tadpoles develop far more slowly, and it now appears 
that at the same time the stage of development of the tadpoles 
differs considerably. On June 26" I found tadpoles of 6—12 m.m.; 
then the development slowly continues; on August 6% 1 found the 
first complete metamorphosis, (the tail has disappeared) the length 
being 15 m.m. On October 10% another tail-reduction takes place. 
Some tadpoles do not metamorphose, they become quite big animals, 
viz. 16—17 m.m., with only short hind-limbs. 
As to the uranium-tadpoles they remain backwards and develop 
far more slowly, which are the most striking characteristics. The 
following table informs us about the size. 
From this table it appears, that the development takes place 
considerably more slowly than with the animals under control, also 
the sizes are smaller. Although the differences seem little, the tad- 
1) Piscidine is a preparation containing dried and powdered fish. 
2) In these and all following measurements the length must be considered as 
taken from the top of the head to the beginning of the tail. 
