From zygote to neural tube. After a number of symmetric divisions, the (mammalian) zygote differentiates into a blastocyst. The outer cells contribute to the placenta and the inner cell mass forms the embryo. The blastocyst then implants itself in the uterus wall and proliferates to form epiblast and hypoblast cells. The former form ectoderm and the amniotic sac, while the latter give rise to the endoderm and the yolk sac. The embryo then elongates after which epiblast cells enter the space between the two layers of the bilaminar disc through an invagination called the primitive streak. These cells then form both mesoderm and, soon thereafter, the neural tube. The mesoderm induces the formation of the notochord, which in its turn induces neuroderm by secreting Chordin. After folding of the neural ridge, the ectoderm closes over the neural tube and neural crest cells migrate to distant locations.
