V

Cut edge of amnion

Prenotochordal cells

-- Cloacal membrane

Primitive node

Primitive streak

Epiblast

Amnioblasts

Primitive node

Primitive streak

Amnioblasts

Epiblast

Prenotochordal Cells

Yolk sac

Invaginating mesoderm cells

Hypoblast

Figure 4.3 A. Dorsal side of the germ disc from a 16-day embryo indicating the movement of surface epiblast cells (solid black lines) through the primitive streak and node and the subsequent migration of cells between the hypoblast and epiblast (broken lines). B. Cross section through the cranial region of the streak at 15 days showing invagination of epiblast cells. The first cells to move inward displace the hypoblast to create the definitive endoderm. Once definitive endoderm is established, inwardly moving epiblast forms mesoderm. C. Scanning electron micrograph through the primitive streak of a mouse embryo showing migration of epiblast (eb) cells. The node region appears as a shallow pit (arrow). D. Higher magnification of the section in C.

Yolk sac

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