MAP OF THE ANLAGE FIELDS IN THE AVIAN UNINCUBATED BLASTODERM

By excision at different sites of rectangular fragments from unincubat ed chicken blastoderms and replacement by isotopic fragments from unin cubated quail blastoderms, we could make the first complete map of the Anlage fields in the freshly laid avian blastoderm. All the Anlage fi elds (Fig. 11) are found in the upper layer (UL) of the caudal half of the area centralis (bordered by the Rauber-Koller's sickle). In the U L of the area marginalis, peripheral to Rauber-Koller's sickle, neithe r gastrulation nor neurulation phenomena could be observed. Similar he terotopic replacement experiments indicate that before incubation, the different parts of the UL of the area centralis are still uncommitted or reversibly committed. The Anlage fields of chordamesoblast and def initive endoderm (gut endoderm) in unincubated avian blastoderms appea red to be disposed caudally in the caudal half of the area centralis. As far as we know we are the first to demonstrate that the Anlage fiel d of the definitive gut endoderm (which is derived from the upper laye r: Hunt, 1937; Vakaet, 1962b) is localized in the most caudal upper la yer part of the area centralis just centrally to the Rauber-Koller's s ickle. The Anlage field of the neural plate is localized in the upper layer over the more cranial endophyll. The Anlage of the brain is shie ld-shaped, whilst the other Anlage fields are sickle-shaped, parallel with the Rauber-Koller's sickle. Their general hemicircular dispositio n and form still seem to reflect (together with the Rauber-Koller's si ckle) the original ooplasmic radial symmetry (Callebaut, 1972) combine d with the eccentricity of the deep layer components, which was observ ed during early symmetrization by gravitational orientation of the egg yolk (Callebaut, 1993a,b). The Rauber-Koller's sickle might be homolo gous with the vegetal dorsalizing cells or centre of Nieuwkoop (1973) in amphibian blastulas.