praty-aṇḍam evam ekāṁśād
ekāṁśād viśati svayam
evam – thereafter; sanātanaḥ – the eternal; viśvātmā – all-pervasive; mahā-viṣṇuḥ – Mahā-Viṣṇu; sahasra-mūrdhā – who has thousands of heads; svayam – personally; viśati – enters; praty-aṇḍam – each egg; eka-aṁśāt eka-aṁśāt – by means of His respective expansions.
This Mahā-Viṣṇu entered into every individual universe as His individual plenary portions, each of which is complete with all His mystic opulences. In other words, within each universe Mahā-Viṣṇu assumes the form of the universal soul with thousands upon thousands of heads.
Mahā-Viṣṇu then enters into those billions of universes in separate individual forms of Viṣṇu, who are called Garbhodakaśāyī Viṣṇu. The meaning of ekāṁśād ekāṁśāt is that an individual plenary portion of Mahā-Viṣṇu enters each womb-like universe.
The Mahā-Viṣṇu who lies down in the Causal Ocean is a plenary portion of Mahā-Saṅkarṣaṇa. In the form of His individual plenary portions, He personally enters into each of the universes that are manifested from Him. Every one of those plenary portions is directly Viṣṇu, and they are similar to Mahā-Viṣṇu in every respect. Each Garbhodakaśāyī Viṣṇu is the aggregate feature of all forms of the indwelling witness.