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

Multiple Choice

During gastrulation in amphibians,the presumptive mesoderm moves into the interior of the embryo by following fibrils of fibronectin.Given what you know about gastrulation,you predict that the fibronectin is most likely 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 blastocoel