involution (in-vol-u's hun). 1

A condition or proce in which nutrition is incomplete, and in which the original form and function are more or le lost. 2. In embryology, a folding in of a membranous structure, as of the epiblast, as a step in the proce of the development of the embryo. 3. The proce of retrograde development, especially of the uterus in the puerperium or of the organs in general in old age. buccal i. In the embryo, the folding in of the epiblast at the angle formed by the cranial flexure which forms the cavity of the mouth, i. of the uterus. The gradual return of the uterus to its normal size after delivery, by a proce of absorption, pituitary i. In the embryo, the ingrowth from the epiblast of the mouth cavity which forms the pituitary body. senile i. The shriveling of an organ from old age. [Lat., involutio, from in, into, + volvere, to turn.]