21. Which one of the following is NOT a typical movement pattern observed during gastrulation?
    (1) Involution (2) Epiboly
    (3) Furrowing (4) Delamination

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Introduction

Gastrulation is a critical phase in embryonic development where the simple blastula reorganizes into a multilayered structure known as the gastrula. This process involves various coordinated cell movements that pattern the embryo’s body plan. Understanding typical gastrulation movements clarifies how germ layers are formed and organized.

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Typical Movements During Gastrulation

1. Involution
   Involution is the inward rolling of an expanding cell sheet over the internal surface of the embryo. It results in cells moving inside while the sheet spreads internally, contributing to the mesoderm and endoderm formation.

2. Epiboly
   Epiboly is the thinning and spreading of epithelial sheets to enclose deeper layers of the embryo. It often involves ectodermal cells expanding to cover the yolk or deeper cells, facilitating surface coverage during development.

3. Delamination
   Delamination involves the splitting or migration of a single cell sheet into two distinct and parallel layers. This movement is especially critical in early developmental steps, such as hypoblast formation.

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Why Furrowing is Not a Typical Gastrulation Movement

• Furrowing generally describes the formation of a constriction or groove, commonly seen during cytokinesis (cell division), where a contractile ring pinches the cell.

• While gastrulation involves grooving, such as the primitive streak or blastopore formation, furrowing as a cellular movement pattern is not categorized with the fundamental morphogenetic movements like involution or epiboly.

• Instead, terms like invagination better describe the inward bending or folding during gastrulation.

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Conclusion

During gastrulation, embryos undergo several fundamental cell movements—involution, epiboly, and delamination—which are crucial for shaping the embryonic layers. Furrowing, while related to cellular mechanics, is not considered a typical morphogenetic movement in gastrulation.

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Final Answer:
(3) Furrowing