Planes of Cleavage
The ova of most of the animal groups (except some specific cases like insects) are spherical or nearly spherical having their own actual centre comparable to earth shape. Similar to north and south poles on earth, the egg has animal and vegetal poles. The yolk platelets have more density than active cytoplasm and are concentrated more towards vegetal hemisphere. Therefore, when the egg lies in any fluid medium (the fundamental feature of most of the eggs even in the apparently terrestrial eggs like those of birds etc.), the vegetal pole tends to face the centre of gravity and animal pole away from it.