39. Dorsal lip of amphibian is equivalent to chick’s
    (1) Hensen’s node (2) Primitive grove
    (3) Animal pole (4) Vegetal pole

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Introduction

Embryonic development in vertebrates showcases conserved organizational centers critical for establishing the body axis and orchestrating gastrulation. In amphibians, the dorsal lip of the blastopore acts as an organizer, inducing embryonic patterning. In birds such as the chick, the analogous structure is Hensen’s node, serving a similar organizer function.

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The Amphibian Dorsal Lip: The Organizer

• Identified by Spemann and Mangold, the amphibian dorsal lip functions as the Spemann organizer, essential for inducing and patterning the embryonic axis.

• Cells in this region involute to form mesodermal tissues like the notochord and prechordal plate.

• The dorsal lip can induce a secondary embryonic axis when transplanted, illustrating its powerful inductive capacity.

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Hensen’s Node in Chick Embryos

• In chick embryos, the primitive streak forms the midline structure where gastrulation occurs, and at its anterior tip is Hensen’s node.

• Hensen’s node is considered the functional equivalent of the amphibian dorsal lip.

• It acts as the site for mesodermal and axial mesoderm ingressions and secretes signaling molecules that organize embryonic patterning.

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Homology of the Organizer Structures

• Both structures orchestrate dorsal-ventral axis formation, mesoderm induction, and neural patterning.

• Molecular markers such as homeobox genes (e.g., goosecoid) are expressed similarly in both regions, supporting their equivalence.

• Transplantation experiments confirm that Hensen’s node can induce secondary axes like the amphibian dorsal lip.

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Conclusion

The dorsal lip of amphibians and Hensen’s node in chick embryos are evolutionary conserved organizer regions essential for early embryonic development. Their functional equivalency underscores shared mechanisms of axis formation and tissue induction across vertebrates.

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Final Answer:
(1) Hensen’s node