The rule is, to each grub one acorn.
Before trusting the egg to the acorn it is therefore essential to subject it to a thorough examination, to discover whether it already has an occupant.

This possible occupant would be at the base of the acorn, under the cover of the cup.

Nothing could be more secret than this hiding-place.

Not an eye could divine the inhabitant if the surface of the acorn did not bear the mark of a tiny perforation.
This mark, just visible, is my guide.

Its presence tells me that the acorn is inhabited, or at least that it has been prepared for the reception of the egg; its absence tells me that the acorn has not yet been appropriated.