During week 3 of human development, the blastocyst is fully embedded in the endometrial tissues, or decidua, and it undergoes a process called gastrulation, which starts around day 14.
During gastrulation, the cells of the blastocyst become reorganized significantly, and by the time the process is finished, it’s no longer a blastocyst at all—it’s a gastrula!
Gastrulation begins with the formation of the primitive groove (sometimes called the primitive streak), a narrow depression that runs down the center of the epiblast layer.
When viewed from above, the groove starts near the tail or caudal end of the embryo, and grows towards the head, or cranial end.
This groove defines the cranial-caudal axis, and the two sides of the groove represent the first instance of bilateral symmetry in the embryo—a left and right side to the body.
Closer to the midline (where the groove is located) is considered medial, and closer to the edges is lateral.
If you view the groove from the side, then you can see that the groove forms on the dorsal, or back, side of the embryo, which makes the dorsal-ventral axis more obvious.
The round bilaminar disc also elongates, and starts to resemble a guitar pick, narrow at the caudal end and wide at the cranial end.
At the cranial end of the primitive groove, a small mound of tissue develops called the primitive node, and a tiny dimple forms within it, called the primitive pit.
The primitive groove, primitive node, and primitive pit together form the primitive streak. Try saying that 3 times quickly...
Okay, so as the primitive streak forms in the epiblast layer, some epiblast cells start to migrate towards the primitive groove, move down into the bottom of the groove, and then actually dive right into it. It’s a bit like a child diving into a ball pit at a funhouse.
The epiblast cells that slip through the primitive groove begin to differentiate to form new cell layers.
Some epiblast cells dive deep and form the embryonic endoderm layer; these freshly differentiated endoderm cells quickly replace the ventral hypoblast cell layer.
Other epiblast cells take a more shallow dive and form the embryonic mesoderm layer, which meets up with the extraembryonic mesoderm layer that was created earlier from the epiblast.
The extraembryonic mesoderm also divides into two layers: parietal mesoderm and visceral mesoderm.
Finally, the epiblast cells that don’t dive into the groove form the embryonic ectoderm layer.