Musar for Shabbat 274:1

This is the secret of the relationship between מילה, removal of the foreskin and פריעה, the splitting and pulling down of the thin membrane beneath the foreskin (which is part of the operation, and without which the rite of circumcision is halachically invalid). Once this has been accomplished the form will resemble its Creator, and man will emerge in the image of G–d. We have discussed elsewhere how different parts of the human body correspond to different regions of the ספירות, emanations. Just as there can be no peace and harmony in the Heavenly Regions until the power of the emanation מלכות has been revealed as the ultimate objective G–d had in mind when He started the creative processes, so man on earth is not "complete" until his member has been revealed minus the foreskin and the membrane underneath it. This is what the sages had in mind when they said מל ולא פרע כאלו לא מל, "If someone removed his foreskin and failed to split that membrane, he might as well not have removed the foreskin at all" (Shabbat 137). Similarly in the case of a Sotah, (a woman who has to disprove accusations of marital infidelity), the Torah demands that the hair of her head be loosened and thereby prominently displayed to indicate that she had committed a violation of religious norms (Numbers 5,18). [It is considered an act of ערוה, unchastity, for a married Jewish woman to thus display her hair. Ed.]