As the Meggid goes on to explain, "It is again the same in the life and growth of any individual human being. The individual--shall I say the 'potency'?--which descends into the embryo and begins its work of informing, finishes the different stages of its work at different times during his life and then is progressively set free . . ." (UV 46).1
It was Steiner's contention that the informing forces are, in a sense, homologous throughout nature, so that there exists a real connection between, for example, the tendency of a river to flow along a convoluted path and the convolutions of the human brain. (See, for example, the Steiner-inspired Sensitive Chaos: Flowing Forms in Water, Wind and Air, by Theodore Schwenk.)
The descent of the potency is perhaps best thought of as "a sort of objective geometrizing, which is engaged in giving the human form its whole shape and quality--and to the brain the configuration which will be required to support it when it later blossoms as conscious activity" (WA 142; Sanderson is speaking).
The descent of the
potency is not an invisible process. As the Meggid points out, it is
really not at all difficult to see that "interior" forces, which have been producing a physical form by incarnating, are set free when their work is completed. It happens . . . over much of the earth every autumn. When the stem and the leaves, and at last the flower, have been given physical form, we glimpse their departure in the revelation of light and colour and scent which ascend from the blossom like a cloud, on their way back to immateriality. You will see also the difference between the only way of thinking that was open to the Gabriel age--and the way which is open to the age which is now upon us. (UV 42)The descent of the potency is operative in the entire biological world, as Barfield reminds in Worlds Apart:
If you really look . . . at a simple phenomenon like, say, a flower and the butterfly hovering over it, it will really tell you of itself that it is not something that arose through the interplay of discrete physical units, but the material manifestation of an immaterial unity from which both flower and butterfly have sprung. An invisible "common ancestor," if you like. (157-58, Sanderson is speaking)