Stem cells: science, policy, and ethics
J. Clin. Invest. Gerald D. Fischbach, et al. 114:1364 doi:10.1172/JCI23549 [Go to this article.]

Figure 1
From zygote to blastula: the early stages of human development. Shortly after fertilization, the zygote repeatedly divides to form a solid mass of cells known as the morula. Two to three days after fertilization, the morula enters into the uterine cavity and forms a hollow sphere: the blastocyst. The surface cells form the trophoblast and give rise to extraembryonic tissues, while the inner cell mass is the source of embryonic stem cells and ultimately gives rise to the embryo, following implantation in the uterine wall.