Rauber's (Koller's) sickle: the early gastrulation organizer of the avian blastoderm.

A fragment of Rauber's (Koller's) sickle of an unincubated quail blastoderm grafted on the ventral side of an unincubated chicken blastoderm (stage 1, Vakaet, 1962) cultured in vitro evokes the formation of a normal secundary primitive streak (PS), mesoderm and definitive (gut) endoderm, derived from the chicken host. The graft itself (containing gamma and/or beta ooplasm, Callebaut 1987) differentiates into the V-shaped junctional endoblast in continuity medially via a transitional endoblast with the median sickle endoblast (Vakaet, 1970). A PS developed also often after grafting of the caudal part of the endophyll, close to Rauber's sickle. However after grafting of the central part of the endophyll (containing delta ooplasm, Callebaut, 1987) no PS appeared. Our study indicates that material from the sickle or its derivatives represent the organizer in the early avian embryo. Comparison with the development in vivo indicated that in the avian gastrula an anchor-shaped connection area exists, situated between the sickle derived deep layer and the structures evoked in the upper layer. Our results suggest the existence of a homology between the avian Rauber's sickle and the blastoporus in amphibians and some reptiles.