When seeds sprouts, the sproutlings will have one (monocots) or two (dicots) leaves. Most of the herbaceous plants and vegetable types in our yards and gardens are dicots, including the cabbages. These initial sprouts grow from the nutrients provide by the seed, activated by light and water. After a few days, these cotyledons will sprout their first set of “true” leaves. These will be shaped more like the mature leaves that the plant will develop (the cotyledons have a few shapes and are all quite small, cabbages have sort of heart shape).
This morning, day 10 of the cabbage sproutlings, I noticed the true leaves are budding. You can just see them at the node between the two first leaves.