Gastrulation is the event occurs in the early developmental stage of most animals. During gastrulation, an embryo, which is constituted by single layered cell sheet, become complicated structure to form three germ layers, accompanied with dynamic cell migration, body-axes determination, and cell differentiation. In a biology textbook, it may be written that ectoderm, mesoderm, and endoderm layers of the three germ layers would be differentiated into neuron and skin, muscle and bone, and internal organs, respectively. However, recent studies challenged for the paradigm and more prolonged cell fate determinations were proposed. In this lecture, we will use mice as model to study about gastrulation, and discuss about stem cells, cell fate determination, an cell differentiation by reading the following paper together.