Primitive streak is initiated and maintained by
Nodal gene
BMP4
FGF
Brachyury gene
Ans. is 'a' i.e. Nodal gene "The primitive streak in itself is initiated and maintained by expression of Nodal, a member of the transforming growth factor b (TGFb) family." - Longman's Medical embryology 10/e, p 57
Fertilization and Development of Embryo