18. Inward movement of an expanding outer layer spreading over the internal surface during gastrulation is termed as
    (1) invagination (2) Ingression
    (3) involution (4) delamination

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Introduction

Gastrulation is a major phase of embryonic development where the single-layered blastula transforms into a multilayered structure called the gastrula. This process involves a series of coordinated cell movements, including invagination, involution, ingression, and delamination. Among these, involution is the migration of an expanding outer cell sheet that moves inward to cover the inner surface of the embryo, playing a crucial role in forming the mesoderm and endoderm.

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What is Involution?

• Involution is the inward rolling or turning of an epithelial sheet around the basal surface.

• Unlike invagination, which is a simple bending inward, involution involves the movement of an entire sheet of cells that spreads over internal surfaces.

• It results in the internalization of cells without individual cell migration.

• This movement helps reorganize cells to form the mesoderm and endoderm beneath the ectoderm.

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Role in Embryonic Development

• In vertebrates like amphibians and fishes, involution is critical for the displacement of the blastocoel by the archenteron (primitive gut).

• The involuting marginal zone cells migrate inside as a sheet, contributing to the inner germ layers.

• It is coordinated with other movements such as epiboly (spread of the ectoderm) and ingression (individual cells detaching and migrating).

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Comparison with Other Cell Movements

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Conclusion

Involution is a vital gastrulation movement essential for shaping the embryo’s internal germ layers. Understanding this process reveals the intricate choreography of cells during early development that leads to the formation of complex organisms.

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Final Answer:
(3) involution