Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 2
कललं त्वेकरात्रेण पञ्चरात्रेण बुद्बुदम् ।दशाहेन तु कर्कन्धू: पेश्यण्डं वा तत: परम् ॥ २ ॥
kalalaṁ tv eka-rātreṇapañca-rātreṇa budbudamdaśāhena tu karkandhūḥpeśy aṇḍaṁ vā tataḥ param
On the first night, the sperm and ovum mix, and on the fifth night the mixture ferments into a bubble. On the tenth night it develops into a form like a plum, and after that, it gradually turns into a lump of flesh or an egg, as the case may be.
The body of the soul develops in four different ways according to its different sources. One kind of body, that of the trees and plants, sprouts from the earth; the second kind of body grows from perspiration, as with flies, germs and bugs; the third kind of body develops from eggs; and the fourth develops from an embryo. This verse indicates that after emulsification of the ovum and sperm, the body gradually develops either into a lump of flesh or into an egg, as the case may be. In the case of birds it develops into an egg, and in the case of animals and human beings it develops into a lump of flesh.