Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.8

8.3.2.8

yadvevaitā atropadadhāti | prajāpatireṣo 'gniḥ saṃvatsara u prajāpatistasya madhyameva madhyamā citirmadhyamasya varṣāśaradāvṛtū tadyadetā atropadadhāti yadevāsyaitā ātmanastadasminnetatpratidadhāti tasmādetā atropadadhāti

And, again, as to why he places them in this (layer),--this Agni (the fire-altar) is Pragâpati, and Pragâpati is the year. Now the middlemost layer is the middle of this (altar), and the rainy season and the autumn are the middle of that (year): hence when he places them in this (layer), he thereby restores to him (Agni-Pragâpati) what part of his body these (formed),--this is why he places them in this (layer).