invagination
Concise Oxford English Dictionary © 2008 Oxford University Press:
invagination/-ˈneɪʃ(ə)n/
▶noun chiefly Anatomy & Biology the action or process of being turned inside out or folded back on itself to form a cavity or pouch.
■ a cavity or pouch so formed.
– derivatives
invaginate verb.
invaginate verb.
'invagination' also found in these Oxford entries:

