The world arises from the imperishable the way a thread arises from a spider — out of it, not out of anything brought to it.The three similes are set against the causal sequence they illustrate. What all three share is that nothing is fetched from outside, which is why they are the similes chosen and not others.
In plain words
Through knowledge-filled Tapas, Brahman swells; from it arises Anna, the unmanifest causal matrix. From Anna come Prāṇa, Manas, and Satya; then the worlds, Karma, and Amṛta, the enduring fruit of Karma.