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unknown between what limits the organism regulates the percentage 
of salt (for this is the principal factor) in the different fluids of the 
body, or by what means it keeps up this percentage. It is certain 
that bony fishes are in general stenohaline, i.e. that each species is 
bound to an osmotic pressure of the water in which it lives, and 
which must not vary too much and especially not too quickly. A 
behaviour like that of the migratory fishes is exceptional. The Baltic 
Sea which contains about all gradations between salt and fresh water 
and the fauna of which has often been studied, furnishes a proof of 
the statements made. Of every species of fish, found in the Baltie 
Sea, I have traced the range of distribution, and the lists, for the 
publication of which we have no room here, show that most fresh 
water fish go some length into the brackish water and most sea-fish 
sustain a certain diminution of the percentage of salt, but that cer- 
tain limits are not exceeded. 
If we want to penetrate into the mechanism of these physiological 
phenomena it is of primary necessity, to know the osmotic pressure 
of the blood of the various species of fish. Some determinations were 
made by Borrazztr and Roper. About five years ago I began to take 
part in these measurements under very unfavourable circumstances 
with sea-fish that had been transported alive from Katwyk to Leyden. 
These animals were mostly alive, in any case entirely fresh. The 
results were not published until in the summer of 1904 they appeared 
to be quite concordant (and sometimes fully to agree) with results 
that had been obtained under more favourable conditions. 
The excellent opportunity of obtaining live sea-fish in great variety, 
afforded by the fishmarket at Bergen in Norway, induced me last 
summer to take up the investigation again and to extend it. After 
that determinations were made on fresh-water fish from the environs 
of Utrecht and finally I was enabled through the kindness of Dr. 
Kersert, director of the zoological garden and aquarium at Amster- 
dam, to study sea and freshwater fish, among these species that are 
difficult to procure. I wish to express here my indebtedness to 
Dr. KeRBERT. 
At Bergen the fish are offered for sale alive in a large num- 
ber of open wooden troughs through which a vigorous current of 
sea-water is passed, which, as I was assured, is pumped up from the 
fjord at a great distance from the town. The fish are not entirely 
normal, however; the catching, the lack of food, the transport, their 
being handled by sellers and buyers, all harm the animals. Before 
they come into the market tanks, namely, they swim in caufs, closely 
packed together in the surface water of the harbour, which sometimes 
