Marginal zone cells--the primitive streak-inducing component of the primary hypoblast in the chick.

(1) Removal of both the area opaca and the marginal zone of the area pellucida from a blastoderm stripped of its hypoblast (type-IV operation) prevents the regeneration of a normally functioning primary hypoblast. (2) Stage-XIII E.G. & K blastoderms (prior to the appearance of PS) after a type-IV operation do not form a primitive streak. (3) In order type-IV operated blastoderms in which the primitive streak has already begun to appear, the regeneration of a non-functional hypoblast did not support the normal maturation of the primitive streak, and in many cases the already existing rudimentary streak was absorbed. (4) Type-IV operated blastoderms from stage 3+ H + H and on developed normally. (5) It is concluded that the cellular contribution of the marginal zone to the primary hypoblast is instrumental in the latter's capacity to induce a PS.