3. 11 Stage XI (ca. 10. 5-12. 5 mm) 48 3. 11. 1 Axial Relations 48 3. 11. 2 Lateral Relations 49 3. 11. 3 Summary . . . . 52 3. 12 Stage XII (ca. 13-16 mm) 53 3. 12. 1 Axial Relations 53 3. 12. 2 Lateral Relations 56 3. 12. 3 Summary 59 4 Discussion . . . . . 60 4. 1 Introduction 60 4. 2 Early Differentiation of the Somite Mesoderm 60 4. 2. 1 Dermatome . . . . 60 4. 2. 2 Myotome . . . . . . . . . . 61 4. 2. 3 Somitic Mesenchyme . . . . . 61 4. 3 Development of the Axial Mesenchyme 63 4. 4 Development of the Somitic Mesenchyme 66 4. 4. 1 Segmentation Process in the Somitic Mesenchyme 66 4. 4. 2 Differentiation of the Somitic Mesenchyme into the Mesenchymatous Primordium of the Axial Skeleton 70 4. 4. 2. 1 Metameric Condensations: Arcual and Costal Processes 70 4. 4. 2. 2 Axial Somitic Mesenchyme (Transverse Commissure) 73 4. 4. 2. 3 Perichordal Tube . . . . . . . . . . . . . . 74 4. 4. 2. 4 Linkage Between Lateral and Axial Segmentation 78 4. 4. 2. 5 Origin of the Mesenchymal Vertebral Bodies . . . 81 4. 4. 2. 6 Blastema of Vertebral Processes and its Relationship with the Blastemic Vertebral Body . . . . . . . . . . . . 83 4. 4. 3 Differentiation of the Somitic Mesenchyme in Relation to the Development of the Peripheral Spinal Nervous System 85 4. 5 Differentiation into Cartilaginous Axial Skeleton . . . . . . 89 . 4. 6 Differentiation of Myotomes; Morphology of the Developing Myotome . . . . . . . . . . . . . . . . . . . . . . . . 92 4. 7 The Notochord . . . . . . . . . . . . . . . . . . . . . 94 Some Remarks on the "Neugliederung" Concept with Special At­ 4.