Shatapatha Brahmana 7.4.1.2

7.4.1.2

sa gṛhṇāti | mayi gṛhṇāmyagre agnimiti tadātmannevāgre 'gniṃ gṛhṇāti rāyaspoṣāya suprajāstvāya suvīryāyeti tadu sarvā āśiṣa ātmangṛhṇīte māmu devatāḥ sacantāmiti tadu sarvāndevānātmangṛhṇīte tadyatkiṃ cātmano 'dhi janayiṣyanbhavati tatsarvamātmangṛhṇīte sa vai tiṣṭhannātmannagniṃ gṛhītvānūpaviśya cinoti paśureṣa yadagnistasmātpaśustiṣṭhangarbhaṃ dhitvānūpaviśya vijāyate

He takes him in (by muttering, Vâg. S. X III, 1), 'Within me I first take Agni,' he thereby first takes Agni into his own self;--'for increase of wealth, for healthy progeny, for vigorous manhood!' and hereby he takes all blessings to himself;--'and may the deities stand by me!' and hereby he takes all the gods to himself; and thus he takes into his own self all that he is about to generate from his own self. Having taken Agni into his own self whilst standing, he builds him up sitting;--Agni is an animal: hence the animal, having received the foetus standing, gives birth after lying down.