Fig. 1.

Avian development before gastrulation. A) Orientation of the embryo on the yolk during intrauterine stages of development. Arrows indicate the direction of rotation of the egg and yolk during the transition through the oviduct. B) Segregation of the embryonic epiblast and extra embryonic Area Opaca. C) Development of the hypoblast. D) Cross sections during intrauterine development. E) Spatial patterning of essential paracrine developmental signals: Wnt8A is initially expressed in the Area Opaca, there is an anterior-posterior gradient of Bmp4 in the Area Opaca, Gdf3 is expressed in the posterior epiblast and Fgf8 is produced in the hypoblast.