40. The grafting of the dorsal lip of the blastopore from an early Xenopus gastrula onto the ventral side of an early embryo will result in two complete embryos. Thus dorsal can be designated as

(1) Primary organizer
(2) Cytoplasmic determinant
(3) Morphogen
(4) Commitment

Introduction

The dorsal lip of the blastopore in Xenopus embryos is a critical structure in early embryogenesis, famously identified as the primary organizer. Its transplantation to the ventral side of an embryo induces the formation of a complete secondary axis, revealing its powerful inductive properties in orchestrating body patterning.

The Spemann-Mangold Organizer

• The dorsal lip was first characterized in landmark grafting experiments by Spemann and Mangold, where transplantation of this tissue induced a secondary embryonic axis in amphibians.

• This region is responsible for signaling and coordinating the development of embryonic germ layers and axial structures such as the neural tube, notochord, and somites.

• The organizer functions by secreting molecules like Noggin, Chordin, and Follistatin that antagonize ventralizing signals (BMPs), allowing dorsal tissues to form.

Mechanism of Organizer Activity

• The dorsal lip suppresses epidermal fates and promotes neural and mesodermal differentiation by modulating morphogen gradients.

• It induces involution and convergence-extension movements essential for gastrulation and elongation of the body axis.

• The organizer also controls spatial gene expression, including key transcription factors like Goosecoid, which further establish dorsal identity.

Biological and Evolutionary Significance

• Understanding the primary organizer has been fundamental in developmental biology, highlighting how localized signaling centers control pattern formation.

• Homologous organizer regions exist in other vertebrates (e.g., Hensen’s node in birds), attesting to the evolutionary conservation of this mechanism.

• Studying organizer molecules informs regenerative medicine and congenital defect research.

Conclusion

The dorsal lip of the blastopore in Xenopus is the seminal example of a primary organizer in vertebrate development. Its ability to induce a complete secondary embryo upon transplantation underscores its central role in embryonic patterning and morphogenesis.

Final Answer:
(1) Primary organizer