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gradually reopens (though to a varying extent in different fish, as might 
be antieipated) until at length it reattains its normal completely open con- 
dition (Text-fig. 3). Judging from my later experiments, the oval is most 
completely closed from 1—2 hours after weighting and presumably 
the oxygen gland is most active at that period. In my earlier ex- 
periments however I assumed that the gland was at its maximum 
activity from 5—6 hours after weighting, but though this assumption 
was apparently not correct, it obviously could not have vitiated my 
Fig. 3. In A the position of the oval in» the dorsal wall of the bladder of the 
Pollack is shown. In B the oval is shown to be widely open but not completely 
expanded; in C the oval is half elosed; in D the oval is almost eompletely closed. 
experiments to any great extent since in fishes which have been 
weighted 6 hours the oval is generally still closed to a considerable 
extent and the fishes have not by this time by any means completely 
regained their normal stationary attitude in the water. 
From the facts just stated, it would seem that the oxygen gland 
is normally in a slightly active condition, since it is constantly pumping 
oxygen into the bladder equivalent to that abstracted by the blood at 
the open oval. All that the gills have to do, under normal conditions, 
is that which respiratory organs in other animals have to do, viz. 
make good the oxygen used up in the oxidation of the tissues. The 
function of the bladder in most teleosts is the maintenance of the 
equality of the specific gravity of the fish with that of its surrounding 
medium; incidentally, however, the bladder serves to some extent as 
a store of oxygen upon which the blood can draw, but this cannot 
possibly constitute the principal function of the bladder, as Mlle. 
Popra (Ann. Sci. nat., Zool., T. 12, 1910) would have us suppose. 
The facts that numerous shallow-water and non-migratory deep-water 
fishes fill their bladder with nitrogen and carbon dioxide and that many 
other fishes possess no bladder at all at once disposes of this view. 
