Mahat Brahma (Nature) is my great womb. There I cast my seed, and hence follows the birth of all beings.
Krishna gives a new connotation for the birth and prevalence of Nature. The whole Nature is not just an aggregate of matter and energy. Like anything born of a womb, the creation also has its limbs with their respective powers and potentials.
There is a presence, power, besides matter and energy, which impregnates the womb of Nature and brings forth the amazing variety of creation. All beings are born like this from the great womb of Nature.
Different beings, born of water, earth and air, have emerged like this. In each species, there is amazing variety, revealing a great order, sequence. The human is the foremost of them all. Krishna strikes a new note here in describing himself (because of his identity with the Supreme) as the progenitor of the whole Creation.