The Compendium of Caraka 1.30.10

1.30.10

yat sāramādau garbhasya yattadgarbharasādrasaḥ /
saṃvartamānaṃ hṛdayaṃ samāviśati yat purā //

Which is the essence at the beginning of the embryo, which is the nutrient fluid of the embryo's nutrient fluid;
which first enters the developing heart;