50 MARVELS OF FISH LIFE 
this time the sparkling water, making its way in little 
eddies among the stones, carries oxygen to the develop- 
ing embryo. 
In six weeks two little black dots appear in the egg, 
which are the eyes of the fish ; and in another six weeks 
the young trout is ready to hatch. But for days before 
this the prisoner had found his quarters too confined, 
and had been wriggling and struggling to get free. After 
a specially violent effort, the egg membrane splits and 
the delicate tail appears. Next the back and the head 
of the fish escape, and finally the encasing “ shell ” slips 
off the yolk sac. Strong, healthy fish usually hatch in 
this manner, but not infrequently the head and shoulders 
appear first, and occasionally the yolk alone protrudes 
as a constricted bladder. When this occurs, hatching 
is always delayed, and not infrequently the young fish 
dies in the egg. On one occasion, while I was watching 
salmon hatch, I picked out an egg with just a round 
bead of yolk sac protruding, and placed the egg in the 
photographic tank; presently the head and shoulders also 
shot through, giving the hatching fish the quaint “ jack- 
in-the-box ” appearance shown on the opposite plate. 
Hatching with the yolk sac nipped in, as shown, took 
a long time, and all the while the young fish kept up a 
continual fanning movement with his pectoral fins, the 
object of which will be referred to later. 
The larva of the salmon family when first hatched 
is known as an “alevin,” the name being derived 
from a French word meaning a young fish. 
