Śrī brahma-saṁhitā 5.14
ekāṁśād viśati svayam
prati — each; aṇḍam — egglike universe; evam — thus; eka-aṁśāt eka-aṁśāt — as His own separate subjective portions; viśati — enters; svayam — personally; sahasra-mūrdhā — possessing thousands of heads; viśva-ātmā — the Supersoul of the universe; mahā-viṣṇuḥ — Mahā-Viṣṇu; sanātanaḥ — eternal.
The same Mahā-Viṣṇu entered into each universe as His own separate subjective portions. The divine portions, that entered into each universe are possessed of His majestic extension, i.e., they are the eternal universal soul Mahā-Viṣṇu, possessing thousands of thousands of heads.
Mahā-Viṣṇu lying in the spiritual Causal Ocean is the subjective portion of Mahā-Saṅkarṣaṇa. He entered, as His own subjective portions, into those universes. These individual portions all represent the second divine puruṣa lying in the ocean of conception and is identical with Mahā-Viṣṇu in every respect. He is also spoken of as the divine guide, from within, of all souls.