Abstract
During the first 4 days of incubation the chick blastoderm expands to surround the yolk. Its expansion takes place over the inner surface of the vitelline membrane, and the edge of the blastoderm is firmly attached to this membrane. Little attention has been paid hitherto to the mechanism of this expansion, presumably because it lies outside the embryo proper. But many of the problems involved are of considerable interest, not only as they relate to development within cleidoic eggs, but also in connexion with more general questions affecting expansion of epithelia and the nature of cell surfaces. Blastoderm expansion has many points of similarity with the spreading of epithelia across wounds, and some of the factors involved may prove to be similar to those affecting the radiation of loose sheets of cells in tissue culture.
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