Only fully methylated origins can initiate replication;

hemimethylated daughter origins cannot be used again until they have been restored to the fully methylated state.

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#Unit 5. Developmental Biology

PMZ = Amphibian Nieuwkoop center

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#Unit 5. Developmental Biology

Primitive Streak

First arises from a local thickening of the epiblast at the posterior edge of the area pellucida (Koller’s sickle)

By stage 2, the primitive streak cells form a third layer that lies between the hypoblast and epiblast cells.

 By stage 3, the primitive streak has become a definitive region of the epiblast, with cells migrating through it to become the mesoderm and endoderm.

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#Unit 5. Developmental Biology

Primitive Groove

A depression that forms within the primitive streak that serves as an opening through which migrating cells pass into the deep layers of the embryo.

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#Unit 5. Developmental Biology

Primitive groove is homologous to the amphibian blastopore

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#Unit 5. Developmental Biology

Hensen’s node (Primitive Knot)

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#Unit 5. Developmental Biology

A regional thickening of cells at the anterior end of the primitive streak