17. Bones of vertebrates embryonic are derived from
    (1) ectoderm (2) epiderm
    (3) mesoderm (4) endoderm

The bones of vertebrates during embryonic development are primarily derived from the mesoderm layer. The mesoderm, one of the three primary germ layers formed during gastrulation, gives rise to most skeletal components, including the axial, appendicular, and craniofacial skeleton. Specifically, mesenchymal cells within the mesoderm differentiate into osteoblasts, which produce bone through processes such as endochondral and intramembranous ossification.

---

---

Introduction

The vertebrate skeleton forms an essential framework that supports the body and protects vital organs. Embryologically, the development of these bones begins early with the formation of the three germ layers: ectoderm, mesoderm, and endoderm. Among these, the mesoderm is chiefly responsible for producing the bones of vertebrates.

---

The Mesoderm and Bone Development

• During gastrulation, the mesoderm forms between the ectoderm and endoderm.

• Mesenchymal stem cells within the mesoderm give rise to osteoblasts, the bone-forming cells.

• Bone development occurs via two main mechanisms:

  • Intramembranous ossification: Direct differentiation of mesenchymal cells into osteoblasts that form flat bones such as those of the skull.

  • Endochondral ossification: Bone develops from a cartilage template that eventually mineralizes and is replaced by bone tissue, common in long bones of the limbs.

---

Skeletal Derivatives of the Mesoderm

• The axial skeleton (vertebrae, ribs, and sternum) originates from paraxial mesoderm (somites).

• The appendicular skeleton (limb bones) arises from the lateral plate mesoderm.

• Craniofacial bones are largely formed by neural crest cells (ectodermal origin) contributing to specific skull bones, but the majority derive from mesodermal mesenchyme.

---

Why Other Germ Layers Are Not Involved

• Ectoderm primarily forms the nervous system, skin epidermis, and associated structures.

• Endoderm contributes to internal linings of the gut, respiratory tract, and some glandular tissues.

• Neither ectoderm nor endoderm contributes directly to vertebrate bone formation.

---

Conclusion

Vertebrate bone embryogenesis is a complex process reliant mainly on the mesoderm, which supplies the osteogenic cells for bone formation. The differentiation of mesenchymal cells within this layer is foundational to the development of the vertebrate skeleton, ensuring structure and functionality necessary for life.

---

Final Answer:
(3) mesoderm