Abstract: Our body is developed from a single cell through a hierarchical series of differentiation. The first differentiation results in the blastocyst formation, and subsequently differentiation result in a hierarchical series of blastocyst-like structures (BLSs). A blastocyst-like structure is composed of a trophectoderm-like structure (TLS) and an inner cell mass-like structure (ICMLS) or ICMLSs, and every BLS is contained in another BLS which is higher in hierarchy than that being contained. An important role of TLS is to make up the local microenvironment for ICMLSs, and TLSs of all the BLSs finally develop into connective tissues or epithelium. ICMLSs are stem cells with different potential. The lower the hierarchy the BLS belongs to, the more differentiated the cells of the ICMLSs of the BLS are. ICMLSs of BLSs belong to the lowest hierarchy are tissue-specific stem cells, they are usually unipotent. This development model can apply to all organisms, it is actually a basic law

Key words: blastocyst, blastocyst-like structures, stem cells