Śatapatha Brāhmaṇa ("of a hundred paths") 7.4.2.26

7.4.2.26

yadveva viśvajyotiṣamupadadhāti | prajā vai viśvajyotiḥ prajā hyeva viśvaṃ jyotiḥ prajananamevaitadupadadhāti tāmanantarhitāṃ retaḥsigbhyāmupadadhātyāṇḍau vai retaḥsicāvanantarhitāṃ tadāṇḍābhyām prajātiṃ dadhātyantarevopadadhātyantareva hyāṇḍau prajāḥ prajāyante

And, again, why he lays down an 'all-light' (brick);--the 'all-light' (brick) is progeny, for progeny is all the light: he thus lays generative power (into Agni). He lays it down so as not to be separated from the seed-shedding (bricks),--the seed-shedders being the testicles, he thus makes the generative power inseparable from the testicles. He lays it down between (those two), for within the testicles progeny is produced.