7.2 The trilaminar germ disk (3rd week)

• Introduction
  
• Formation of the primitive streak
• Genesis of the germ layers
• Genesis of the notochord
• Location of the epiblast cell target and the development of the primitive streak
• The bilaminar membranes
• Evolution of the mesoblast
  • The paraxial mesoblast and the differentiation of the somites
  • The intermediate mesoblast
  • The lateral mesoblast
• The intraembryonic coelom
• Induction of the neural plate - neurulation

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 10 and cloacal membrane (membrana cloacalis) 6 caudally. Both of these membranes will be resorbed during the subsequent development. There the openings for the stomodeum11 and of the urogenital tract (cloacal membrane) 19 are located.

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.