Experiments involving transplantation of the roof of the blastocoel in Rana pipiens embryos reveal that the inner surface of the roof of the blastocoel must be coated with a fibrous extracellular matrix (F-ECM) to serve as a substratum for mesodermal cell migration. When the roof of the blastocoel is inverted the original outer surface, now projecting toward the blastocoel, does not become coated with F-ECM and does not support mesodermal cell migration. When the roof of the blastocoel is removed from a normal embryo and transplanted into an interspecific arrested hybrid embryo known to be deficient in F-ECM synthesis, the grafted ectodermal fragment does not become coated with F-ECM and does not support normal mesodermal cell migration. When a hybrid graft is placed in a normal embryo, the grafted ectodermal fragment becomes coated with F-ECM and supports mesodermal cell migration. In normal control embryos migrating mesodermal cells are polarized due to formation of lamellipodia on their leading but not their trailing edges. These cells are arranged in overlapping layers. The leading cells form lamellipodia on the roof of the blastocoel and trailing cells form lamellipodia on one another.

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