In either case, cells from the outside of the embryo migrate into the blastocoel to become endoderm and mesoderm.  
In lower animals, it begins with an involution of the vegetal pole at the dorsal lip.  In mammals, it begins with a thickening of the posterior epiblast and formation of the primitive streak.  In either case, cells from the outside of the embryo migrate into the blastocoel to become endoderm and mesoderm.  FTP, name this developmental process immediately following blastula formation.
In lower animals, it begins with an involution of the vegetal pole at the dorsal lip.  
gastrulation