The retention of primary hypoblastic cells underneath the developing primitive streak allows for their prolonged inductive influence.

An experimental study was made of the distribution of the primary hypoblastic cells in the lower layer of the avian blastoderm throughout primitive streak formation and until stage 10 (Hamburger & Hamilton, 1951). The primary hypoblast of stage XIII (Eyal-Giladi & Kochav, 1976) chick blastoderms was exchanged for either an [H3]thymidine-labelled similar chick hypoblast, or a quail primary hypoblast. During the entire period of primitive streak formation, the lower layer proved to be a mosaic of labelled hypoblastic and non-labelled entodermal cells (chick cells of epiblastic origin). The persistance of hypoblastic cells underneath the developing primitive streak is regarded by us as a possible way to prolong the inductive influence of the hypoblast upon the forming streak.