ABSTRACT
Chick blastoderms during the first 2 days of incubation are unable to expand if the blastoderm edge is detached from the vitelline membrane. The edge can, however, reattach, and then expansion continues.
The blastoderm edge is very sensitive to the nature of the substrate surface. It did not form normal adhesions with, or expand normally over, vitelline membranes that had been boiled or (with one exception) treated with formalin. Neither will it expand over the outer surface of the vitelline membrane (i.e. the surface which in ovo faces the albumen). It will, however, expand over alcohol-treated vitelline membranes.
The blastoderm is sensitive to lines of tension in the vitelline membrane, extending fastest along lines of greatest tension.
The marginal cells of the blastoderm adhere to the vitelline membrane as a result of their intrinsic properties, and not because of their peripheral position.
Expansion of the blastoderm is dependent on a mechanical tension produced by the outward-moving margin.
Expansion can occur without protein synthesis, but if yolk protein is available it will be utilized. Albumen protein is not used at this stage.
Cell proliferation is particularly active in the marginal region of the blastoderm (though not at the extreme edge). The ectoderm in the marginal region becomes several cells thick if blastoderm expansion is prevented.
Blastoderms inverted on the vitelline membrane develop into hollow vesicles.
Blastoderms grown on inverted vitelline membranes adhere abnormally strongly to the membranes and fold the membranes at the region of attachment.
