T04 MARVELS OF FISH LIFE 
fectly perpendicular position with their heads in con- 
fact with the surface of the water. 
During this time, just above the centre of the yolk 
sac, a number of dark-coloured cells have massed 
together in a space where the gas bladder is going to 
appear. These dark cells then draw upwards towards 
the top of this space, leaving the rest of it clear. Sud- 
denly, in this space will appear a minute gas bubble 
caused by the action of these cells on the blood of the 
fish, which rapidly increasing in size, gives the fish the 
appearance of having a pearl in his body. The larva 
now has a primitive gas bladder filled with a bubble of 
oxygen, and immediately his whole mode of life alters. 
He now never goes down to the bottom, but floats in 
the water from six inches to a foot below the surface. 
The manner of progression also changes, and the young 
fish advances with a peculiar jerky movement like a 
water flea, alternately stopping and shooting forward, 
and at night he rests floating in a horizontal position. 
All these changes occur in two to three days. 
The larve henceforward, like the parent fish, swim 
in shoals, keeping in the sunlight, and carefully avoid- 
ing dark corners. The perception of these little creatures 
is extraordinary, and once the gas bladder has appeared 
they are extremely difficult to catch, persistently dodg- 
ing the end of the pipette used for picking them out of 
the water. Even when the pipette is held quite still in 
the hope that one will come sufficiently near to be 
sucked up, they give it a wide berth. 
