We form something called the mesoderm. So that's the middle layer of that developing embryo. And these are the retails, as shown in this diagram. Now, what happens is if we zoom in on this section, we get the following diagram. And what happens is we have this invagination process where the blue layer basically invaginates and informs the primitive streak. And along the primitive streak, which is basically this access where invagination takes place, these blue cells essentially invaginate and move inwards.