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 intermediate mesoblast

The intermediate mesoblast 10 is found between the paraxial mesoblast and the lateral plate mesoblast. This longitudinal, dorsally lying crest is called the urogenital crest and serves as the origin of the kidney and gonads. It forms cell masses in the neck and upper breast regions that exhibit a metamerism: the nephrotomes.
In the more caudally lying regions it remains unsegmented and forms the so-called nephrogenic cord.

The lateral mesoblast

The lateral plate mesoderm is composed of two thick layers that surround a cavity, the intraembryonic coelom (the coelom represents the future serous cavity of the trunk: peritoneal, pleural and pericardiac cavities).
The somatopleure, which is close to the ectoderm, is involved in the formation of the lateral and ventral walls of the embryo.
The splanchnopleure, which lies on the endoblast, takes part in the formation of the wall of the digestive tube.

Fig. 17
After the 23rd day coelomic vacuoles form in the lateral plate.

Fig. 18
On the 25th day the intraembryonic coelom divides the lateral plate into the splanchnopleural and somatopleural mesoderm.