Embryo Week 3

Embryo week 3 is a pivotal period of gastrulation where the primitive streak appears on the epiblast, cells ingress to form mesoderm and definitive endoderm, displacing hypoblast, while the notochord induces neural plate formation from ectoderm beginning neurulation. The allantois and connecting stalk emerge, cardiac progenitor cells migrate, and somites start segmenting; this transforms the flat disc into a structured embryo with axis and basic germ layers, incredibly rapid development where major body plan defects like spina bifida can originate if disrupted.