Transformation of the blastocyst into the gastrula with primary germ layers by rearrangement of the cells is called gastrulation. In human, the germ layers are formed so quickly that it is difficult to determine the exact sequence of events. According to some embryologists, the hypoblast is termed the embryonic endoderm the first germ layer to be formed. Some workers called epiblast as ectoderm the second germ layer.