Gastrulation inducing potencies of endophyll and Rauber's sickle in isolated caudocranially oriented prestreak avian blastoderm quadrants (or fragments) in vitro.

Unincubated quail and prestreak chicken blastoderms were sectioned into cranial, caudal and lateral quadrants and cultured in vitro. Whilst in the caudal and lateral quadrants (containing both endophyll and Rauber's sickle material) a primitive streak (PS) developed, in the cranial quadrants (where merely central endophyll is present) only an ectoblastic thickening (early gastrula: phase I of gastrulation) appeared. If, however, sickle material was grafted on a cranial quadrant then a PS developed (phase II of gastrulation). In the anti-sickle region (lacking sickle material and endophyll) no gastrular differentiation occurred spontaneously. The grafting of endophyll and/or Rauber's sickle evokes here also phase I or II of gastrulation. When the endophyll was removed from the caudal quadrant no PS appeared. If, however, endophyll was replaced, a PS and neurula developed. Our study demonstrates that the presence of endophyll is necessary to obtain an early gastrular development (phase I of gastrulation). It seems also to have an influence on the formation of the future neurectodermal region by induction of a thickening of the cranial upper layer. Only when both endophyll and sickle material were present, a PS formed in caudal quadrants. In the caudal quadrant, the endophyll (which seems to represent the vegetative pole of the avian blastula) does not regenerate, neither from the upper layer, nor from Rauber's sickle (containing blastoporal material: Callebaut and Van Nueten: 1994).