In warm blooded creatures, the approximately 50–150 cells that make up the inward cell mass during the blastocyst phase of early stage advancement, around days 5–14, have undifferentiated cell ability. In vivo, they in the end separate into the entirety of the body's cell types (i.e., they are pluripotent). This procedure begins with the separation into the three germ layers – the ectoderm, mesoderm and endoderm – at the gastrulation stage. In any case, when they are disengaged and refined in vitro, they can be kept in the immature microorganism stage and are known as undeveloped undifferentiated cells (ESCs)."