1. Formation and Differentiation of Avian Somite Derivatives........ 1 Bodo Christ and Martin Scaal Abstract................ 1 Introduction ................. 1 Dermomyotome................ 2 Sclerotome................. 14 Outlook............. 29 2. Avian Somitogenesis: Translating Time and Space into Pa ttern.......... 42 Beate Brand-Saberi, Stefan Rudloff and Anton J. Gamel Abstract............. 42 Introduction.............. 42 Epithelialization of the segmental plate............. 43 The anterior posterior polarization of the paraxial mesoderm.............. 46 Resegmentation of the somitic derivatives.............. 49 Regionalization of somites and segmental plate............... 49 Oscillations in gene expression underlying somitogenesis........... 50 Conclusion and future considerations.............. 52 3. Genetic analysis of somite formation in laboratory fish models........... 58 Christoph Winkler and Harun Elmasri Abstract............. 58 Introduction........... 58 Genetically dissecting the clock in zebrafish: The Delta/Notch somitogenesis mutants.............. 60 Dissecting the wave front in zebrafish: FGF signalling and Tbx24............ 62 Other pathways implicated in somitogenesis............ 64 Medaka: a model complementary to zebrafish............ 64 Somite formation in the teleost medaka............. 65 Medaka somitogenesis mutants............. 66 Medaka somite mutants with PSM prepatterning defects............ 67 Medaka mutants with defective somite polarity............. 67 Conclusions and outlook............. 69 4. Old wa res and new: five decades of investigation of somitogenesis in Xenopus laevis......... 73 Duncan B. Sparrow Abstract............. 73 Introduction............ 73 Structure of the PSM............. 76 Morphological descriptions of segmentation in Xenopus............. 77 A comparison of Xenopus segmentation with that of amniote vertebrates.............. 78 What controls where the somitic furrow forms?.............. 78 Evidence for segmental Prepatterning of the PSM............. 79 Cycling genes-evidence of a "clock"?............. 81 Embryological insights into the nature of the "wavefront"............... 85 The molecular nature of the wavefront............ 86 What are the morphomechanical mechanisms required for somite separation?............. 87 Ena/VASP................ 89 Conclusion.............. 90 5. Role of Delta-Like-3 in Mammalian Somitogenesis and Vertebral Column Formation...... 95 Gavin Chapman and Sally L. Dunwoodie Abstract............. 95 Introduction.............. 95 Somitogenesis............. 95 The Notch Signalling Pathway........... 97 Notch Signalling and Somite Formation.......... 99 Dll1 and Dll3 Perform Different Functions during Somitogenesis in Mammals....... 102 Dll3 Conclusions and the Future........... 106 6. Mesp-Fa mily Genes are Required for Segmental Pa tterning and Segmental Border Formation.........113 Yumiko Saga and Yu Takahashi Abstract.......... 113 Introduction............ 113 Background of Mesp1 and Mesp2........... 114 Function of Mesp2 during Somitogenesis........... 114 Mesp2 Is also Involved in the Segment Border Formation.......... 118 Regulation of Mesp2 Expression during Somitogenesis........... 120 Functional Redundancy between Mesp1 and Mesp2........... 120 Mesp Genes in the Other Vertebrates......... 121 Perspective........... 122 7. bHLH Proteins and Their Role in Somitogenesis........... 124 Miguel Maroto, Tadahiro