During Gastrulation in Amphibians,the Presumptive Mesoderm Moves into the Interior

During gastrulation in amphibians,the presumptive mesoderm moves into the interior of the embryo by following fibrils of fibronectin.The fibronectin is probably secreted by which cells?

A) endodermal cells on the exterior of the embryo
B) mesodermal cells that develop on the inside of the blastocyst
C) ectodermal cells that line the inside of the blastocoel
D) endodermal cells that migrate to the inside of the blastoceol