Jaiminīya (Talavakāra) Brāhmaṇa 1.71

The sadas hall is indeed this belly of Prajāpati;
the udumbara is vigor.
When the audumbarī post is erected in the middle of the sadas hall, he thus places food and vigor in the middle of creatures.
Therefore food, when eaten, nourishes from the middle.
Therefore, wherever this [post] is rendered stale, there the creatures become hungry;
yet they do become thriving.
They deliberate: 'Should it be touched or not touched?'
Everything here is endowed with self.
Whoever lusts after food, he hates that [food] most.
If he were to touch it, food would disperse;
if he were not to touch it, he would separate himself from food.
Having touched it, he should sing with it untouched: thus food does not disperse, nor does he separate himself from food.
The sāman is the food of the gods, the udumbara is vigor.
When the Udgātṛ leans against the audumbarī post, he thus places vigor, the food of the gods, into the sāman.
Therefore, where they perform the priestly office with the sāman, there the gods eat the most, as it were.

Prajāpati is the Udgātṛ; the udumbara is vigor.

Resting on vigor, this Prajāpati distributes vigor and edible food to creatures.

Seated facing north, he sings the udgītha.

He thereby bestows vigor upon the northern quarter.