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Fig. 2. Evidence for an inhibitor of primitive streak formation. (A,B) When a
pellet of chick Vg1-expressing cells is implanted into the lateral margin of a
chick embryo (A), a single primitive streak often forms, arising from the site
of grafting and inhibiting the original streak (B). (C-H) Timed implantation
of two chick Vg1 pellets (C) allow estimation of the speed of travel of the
inhibitor. When implanted 0 hours (D) and 4 hours (E) after the first, the
second pellet also induces a primitive streak. When implanted 6 hours (F,H)
after the first, the second pellet does not induce a primitive streak. When
two axes form, the tips of both primitive streaks express Chordin
(E,G), the body of the primitive streak expresses brachyury (D,E) and
the hypoblast (expressing Hex) is displaced along the axis of each
streak (G,H). p, posterior.
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