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The bilaminar membranes

Two regions exist where the ectoderm and endoderm lie directly on each other without any mesoderm cells lying between them. These two rounded regions are found along the median line. One lies cranially to the prechordal plate, the other caudally to the primitive streak.

During the course of the 3rd and 4th weeks, the ectoderm in these regions bonds tightly with the endoderm lying below it in order to form a bilaminar membrane that is called a pharyngeal membrane (membrana oropharyngea) cranially (stage 10) and cloacal membrane (membrana cloacalis) (stage 6) caudally. Both of these membranes will be resorbed during the subsequent development. There the openings for the stomodeum (stage 11) and of the urogenital tract (cloacal membrane) (stage 19) are located.

Fig. 12 - Dorsal view of the embryo3rd week

  1. Primitive groove
  2. Primitive pit
  3. Primitive node
  4. Oropharyngeal membrane
  5. Cardiogenic plate
  6. Sectional edge of amniotic membrane
  7. Mesoderm
  8. Endoderm
  9. Future cloacal membrane

Fig. 12

Schematic diagram of the dorsal view of an embryo during the mesoblast emigration (3rd week).
To be seen are the two bilaminar regions at the cranial and caudal extremities.

The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.

NB 1+2+3 = primitive streak