It does not hatch but remains in the capsule. The embryo secretes a hatching enzyme. The enzyme digests the capsule from the inside. This enzyme weakens the capsule until it ruptures to free the now very active embryo. Once hatched, the embryo with its large yolk sac is called an alevin.
Hatching usually is rapid once the capsule ruptures.
However, all eggs do not hatch at one time, even though they have had identical temperatures. In a hatchery tray of eggs incubated at one temperature, the eggs may hatch over a period of a few days or more. This table shows embryonic development in the sockeye egg. As the embryo grows it starts to move as well. The first thing that is usually seen moving on an embryo is the heart - more accurately, the heart sac.