Abstract: The ontogenic law is also the law of stem cell differentiation, which may provide the guidance for stem cell research. According to this law, stem cell differentiation  usually results in formation of blastocystlike structure (BLS), which is composed of a trophectodermlike structure （TLS）and an inner masslike structure (IMLS) or IMLSs, and this process repeats circularly. TLSs will finally develop into connective tissue or epithelium tissue. Niches, which somatic stem cells reside in, are attachments of tissue derived from TLSs. Mesenchymal stem cells (MSCs) are progenitors of several connective tissue cell lineages, they are distributed widely in the body. Characteristics of MSCs differ among laboratories, species and tissues, and there is no specific marker or combination of markers that identify MSCs either in vivo or in vitro. In view of the ontogenic law, MSCs should be IMLSs except for that of the lowest hierarchy BLSs, this is the reason why MSCs are distributed so widely and their markers are various.

Key words: stem cells, mesenchymal stem cells, differentiation, blastocystlike structure, trophectodermlike tructure, inner masslike structure