Invagination in Biology
- Invagination is the initial step of gastrulation, which is the massive reorganization of the embryo from a simple spherical ball of cells into a multi-layered organism with differentiated germ layers.
- Invagination also occurs later in embryonic development, such as the invagination of the inner membrane of a mitochondrion to form cristae, providing a larger surface area for ATP production.
- Invagination is involved in endocytosis and exocytosis when a vesicle forms within the cell and the membrane closes around it.
- Intussusception is the invagination of a part of the intestine into another part.
- Invagination plays a crucial role in the morphogenetic processes of embryonic development.

Invagination in Amphioxus
- Invagination in Amphioxus is the first cell movement of gastrulation.
- During gastrulation, the blastula undergoes invagination, transforming into a cup-shaped structure with a double wall.
- The inner wall, called the archenteron, becomes the primitive gut, while the outer wall becomes the ectoderm.
- The archenteron opens to the exterior through the blastopore.
- The invagination process in Amphioxus is described as the folding of the endoderm towards the inner part of the embryo.

Invagination in Tunicates
- Invagination is the first mechanism that takes place during gastrulation in tunicates.
- The invagination process involves internal movements of the endoderm cells, resulting in changes in their shape.
- The blastula of tunicates undergoes a change of shape from columnar to wedge-shaped during invagination.
- Once the endoderm cells are invaginated, they continue to move beneath the ectoderm.
- The invagination process in tunicates is complete with the formation of the blastopore surrounded by the mesoderm.

Invagination in Geology
- In geology, invagination refers to a deep depression of strata.
- Donald L. Baars used the term invagination to describe such depressions in The Colorado Plateau.
- Invagination in geology is a significant geological feature.
- It is used to understand the structural characteristics of rock formations.
- The study of invagination in geology provides insights into the geological history of an area.

References
- Gilbert, Scott; Rauno, Anne. Embryology, Constructing the Organism.
- Gilbert, Scott; Barresi, Michael. Developmental Biology.
- Davies, Jamie A. Mechanisms of Morphogenesis.
- Alberts. Molecular Biology of the Cell.
- Cronk, Jeff. Biochemistry Dictionary.