It behooveth man, upon reaching the age of nineteen, to render thanksgiving for the day of his conception as an embryo. For had the embryo not existed, how could he have reached his present state?

The Báb: Selections from the Writings of the Báb, p. 89

That is to say, the embryo passes through different states and traverses numerous degrees, until it reaches the form in which it manifests the words "Praise be to God, the best of Creators," and until the signs of reason and maturity appear. And in the same way, man's existence on this earth, from the beginning until it reaches this state, form and condition, necessarily lasts a long time, and goes through many degrees until it reaches this condition. But from the beginning of man's existence he is a distinct species. In the same way, the embryo of man in the womb of the mother was at first in a strange form; then this body passes from shape to shape, from state to state, from form to form, until it appears in utmost beauty and perfection.

‘Abdu’l-Bahá, Some Answered Questions, p. 183-184

The embryo in the womb of the mother, gradually grew and developed, and passed from one form to another, from one shape to another, until he appeared with this beauty and perfection, this force and this power. It is certain that in the beginning he had not this loveliness and grace and elegance, and that he only by degrees attained this shape, this form, this beauty, and this grace. There is no doubt that the human embryo did not at once appear in this form, neither did it then become the manifestation of the words: "Praise be unto God,   the best of Creators."

‘Abdu’l-Bahá, Bahá’í World Faith, p. 299