Invagination

invagination [in‚vaj·ə′nā·shən]

(embryology)

The enfolding of a part of the wall of the blastula to form a gastrula.

(physiology)

The act of ensheathing or becoming ensheathed.

The process of burrowing or enfolding to form a hollow space within a previously solid structure, as the invagination of the nasal mucosa within a bone of the skull to form a paranasal sinus.

---

Invagination 

(1) In embryology, in-pouching, one of the processes of gastrulation, in which part of the wall of the embryo turns into its cavity and forms an inner layer, called the primitive entoderm.

(2) In medical practice, a form of intestinal obstruction in which a section of the gut works its way into (usually) a lower section, producing blockage of the lumen of the intestine. It is most often found in children.