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. This heart sac starts to contract. Then slowly it starts to move other parts of the body.
Twitches are evident throughout the embryo.
After the twitching of the body itself the tiny developing pelvic and pectoral fins begin twitching. The fanning motion of the fins is very important to the development of the embryo. The hatching process is a result of a variety of activities that the embryo is going through. In addition to these activities the chemical and enzymatic readiness of the embryo to hatch is crucial to a successful hatch. The embryo grows to such an extent that it needs more and more oxygen until finally it becomes desperate in its oxygen use.