Mesenchymal-epithelial interactions are known to be of utmost importance during embryonic
development. Migration of cells through the blastopore (amphibians) or through the primitive
streak (higher vertebrates) generates the primary mesoderm, part of which, the paraxial
mesoderm and associated notochord, induces the body axis and neural development in the
overlying ectoderm (Sax6n and Toivonen, 1962; Hara, 1978; Takaya, 1978; Toivonen,
1978). During primary induction, regional differentiation is specified cranio-caudally along …