The germ layers form during gastrulation of the blastula. In many animals, the morula then develops by cavitation to become the blastula. In mammals the blastula is referred to as a blastocyst. In others, normal segmentation occurs in the blastula but gastrulation fails. Further divisions occur until "P. flava" finishes the blastula stage and goes on to gastrulation. During gastrulation, the blastula will be transformed by the invagination. Important to the sea urchin blastula is the ingression of the primary mesenchyme. It is the animal hemisphere of the blastula that will eventually become the ectoderm. Most starfish embryos hatch at the blastula stage. After the 7th cleavage has produced 128 cells, the embryo is called a blastula. Only when the blastocoel is formed does the early embryo become a blastula. In vitro fertilisation involves implantation of a blastula into a mother's uterus. Within the blastula, inner blastomeres are generally non-polar while epithelial cells demonstrate polarity. Cleavage ends with the formation of the blastula. Cell-cell interactions mediated by E-cadherin are crucial to blastula formation in many animals.