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reduction, by our perfusion experiments is but small. To grasp this 
it must be remembered that the quantity of artificial urine obtained from 
our experiments with frogs was only 0.1 c.c. Let us take an arbitrary 
example to see what influence a small error has on the titration. 
In table I perfusing with 0.15°/, galactose for obtaining the final 
reaction. was used: 
right kidney 1,83 ce. thiosulphate 
for the urine of frog A 
| left kidney 1,81 „ 
| right kidney 1,83 ,, i 
for the urine of frog B 
| left kidney” 1,80 ,, 
From these figures the value calculated for the galactose retention 
for frog A is 55°/, and 49°/, and for frog B 55°/, and 46°/,. An 
error of 0.02 °/, thiosulphate, therefore, causes an error of 55—49 — 6 °/, 
in the retentive power found. 
To increase the degree of accuracy it is necessary to experiment 
upon larger animals, which supply more urine, thus with kidneys 
of warm blooded animals. For this however a room is necessary 
which can be brought up to body temperature, which, under present 
circumstances, is impossible. 
Be it as it may, if the differences between the retained and the 
not retained had been greater — for, did we not find that approxi- 
mately half was retained and half not —? and, to correspond with 
this, the difference between the quantities of « and B modifications 
in the galactose solution had also been greater, then it would at once 
have been obvious which form is retained and which is not. We 
will return to this in connection with xylose. 
Before we proceed to discuss the behaviour of xylose, we wish 
to face an objection which may, on superficial inspection, be made 
against our representation. It could be remarked that when one of 
the two modifications has been removed by the kidney, from the 
second modification which remains in circulation, more of the first 
modification will be formed, and that eventually all the galactose 
will leave the kidney in that way. Let it be taken into consideration 
however that in our perfusion experiments the perfusion liquid forms 
only a very small quantity of urine and therefore only very little 
of the one kind of galactose is removed. Let it further be femembered 
that, unlike in the normal body, the same perfusion fluid does not 
remain in circulation. In our experiments the solution leaving by the 
renal vein does not return again by the renal artery. 
But what can be the reason then that in the normal organism 
