Selective affinity is the process in which cells position themselves in such a way as to reveal their eventual embryonic positions. This is seen by the three embryonic germ layers known as endoderm, mesoderm and the ectoderm. During development, The inner surface of the ectoderm has a positive affinity for mesodermal cells and a negative affinity for the endoderm, while the mesoderm has positive affinities for both ectodermal and endodermal cells. Therefore, when the three different embryonic cells are all mixed up together, they are seen to separate by themselves using their positive and negative affinities to rearrange themselves in a position that the mesoderm will find itself in between the ectoderm and endoderm, and making sure the ectoderm and endoderm never really connect. This was proven by Boucaut, who injected individual cells from specific germ layers into the body cavity of amphibian gastrulae.
