The embryo of an animal first looks like a hollow sphere. Invaginations then appear at different stages of development, which will give rise to the body’s structures (the brain, digestive tract, etc.). According to a hypothesis that dates back more than a century, buckling could be the dominant mechanism that triggers invagination — buckling being a term that describes the lateral deformation of a material under compression. Although this explanation has long won the support of biologists, it has never been subjected to formal proof, mainly because of the difficulty — if not the impossibility — of measuring the tiny forces involved.
