A turning or rolling in.

Gastrulation by ingrowth of blastomeres around the dorsal lip.

1. Any transformation that is its own inverse.

2. In particular, a correspondence between the points on a line that is its own inverse, given algebraically by x' = (ax + b )/(cx - a ), where a² + bc ≠ 0.

3. A correspondence between the lines passing through a given point on a plane such that corresponding lines pass through corresponding points of an involution of points on a line.

1. The retrogressive change to their normal condition that organs undergo after fulfilling their functional purposes, as the uterus after pregnancy.

2. The period of regression or the processes of decline or decay which occur in the human constitution after middle life.